Could Britain defend Canada from a United US with a Civil War level effort around ~1860?

[Jiraiyathegallant]

To state the obvious, the American Civil War was devastating. The Union and Confederacy had over a million and a half total men killed, wounded, and captured.

Creating a more United US is not easy, but can happen. If we have a variety of PoD resulting in a peaceful emancipation of slaves in most of the Upper South starting in the 1830s, probably led by Virginia, slightly better Compromises than 1820 and 1850, earlier Boll Wevill, or a delayed Cotton Gin, slavery can be weakened and tension leading to a Civil War can be done away with.

If a president who secretly wants all Canada sabotaged the negotiations with Britain over the US/BE Border, we could have a fight break out around the Civil War era over the unresolved tension.

If we had an initially united United States around the time of the Civil War go up against Britain over the border, in a scenario where both sides consider themselves the defender, can the US beat Britain if it has an American Civil War Level effort?

 

[EnglishCanuck]

So what do you mean by 'Civil War Level effort?' The US mobilizing 500,000 men in 6 months? The US willing to pour four years of blood and toil into the conflict? There's a real lack of qualifiers here.

I personally am prepared to say that if the US circa-1860 were willing to pour four years of united national effort into the conquest of Canada, the consequences be damned, they could probably overrun it within two to three years insofar as Canada West, Canada East and New Brunswick are concerned. Nova Scotia though, I would say would be impossible to take and even New Brunswick would probably have to be returned at the negotiating table.

If the US is willing to lose 800,000 dead and all the economic dislocation war with Britain entails, then sure they could invade and conquer Canada. Why they would want such a thing I can't explain but if they were willing to do that, then yes.

 

[RodentRevolution]

A lot depends on how many Presidents in a row and for that matter Congresses were sold on the scheme to expand north. OTL the US on the eve of the American Civil War was a military pygmy. The Army had just under 200 companies spread across nearly 80 army posts all told around 16,000 men. The States militias were nominally larger but lacked modern arms, much training nor possessed much in the way of co-ordination. Of some 90 ships nominally available to the USN most were in ordinary and many besides were sailing ships.

The problem is that was OTL, such radical changes as you suggest are going to alter American attitudes to its military preparedness entirely. For example no slavery in the 1830s means most likely no Texian independence (because why bother?) which leads to no Mexican-American War. It will also have all sort of ramifications for the southern planter aristocracy. I could see if they preserve slavery they might industrialise using slave factory labour, though that would exacerbate tensions with the north most likely. Otherwise the a big chunk of the US elite just got a lot poorer and someone needs to find some kind of productive work for several million ex-slaves, which could cause issues.

However on the British side given such an early POD does the Crimean War and the Indian Mutiny still happen as without these the British Army at least is going to be both numerically weaker than OTL and badly in need of reform.

So if you have a US prepared to spend many years preparing for a Canada expedition you can easily have the situation described by EnglishCanuck above. However without you need some really significant changes to one or both sides to make it other than a humiliating reprise of 1812.

 

[Marquis]

Every US coastal city gets bushwacked by cannonade from a very cross RN battlefleet. The US merchant fleet is hounded to the ends of the earth and back.

The advance into Canada is stymied by cold, terrain, supply trains and guerilla warfare, the American Indians get nice new rifled muskets from their northern neighbours.

 

[FillyofDelphi]

Every US coastal city gets bushwacked by cannonade from a very cross RN battlefleet. The US merchant fleet is hounded to the ends of the earth and back.

The advance into Canada is stymied by cold, terrain, supply trains and guerilla warfare, the American Indians get nice new rifled muskets from their northern neighbours.

This is 1860's. Amerindian tribes anywhere east of the Mississippi are basically not a thing outside Minnesota (Which could get complicated) while those on the British side of the border aren't reliable allies. Virtually all targets sans Halifax worth spit for the Americans are on or near the Great Lakes system, easing logistics. Granted, this does not make the affair easy or a smart idea (The naval war is Britain's, and the US has to crank up to fill some big supply gaps), but I don't think the Canadians are exactly going to be able to put up a robust land defense once the US gets its pistons firing. Particularly if this is pre finalization of Confederation and the war effort is disjointed as a result.

Of course, said period of build up will take at least a year and involve learning all the hard lessons and sorting out needed reforms you get from building an army up. But that applies to both sides, unless and until some officers who learned a thing or two in the Crimea apply.

 

[Fabius Maximus]

Another point worth mentioning is that when the Union Army expanded in IOTL's US Civil War, domestic industry was unable to supply all its weaponry needs, and the country instead made up the shortfall by purchasing equipment from -- the UK. So unless TTL's US gets its industry on a war footing before provoking the conflict, they're going to find it very hard to expand their military to a size capable to taking Canada, or even defending home territory from the British.

 

[Dan1988]

Question - how are you defining Canada here? Are you referring to just the (United) Province of Canada, or the area that makes up OTL Canada today (so all provinces and territories)?

It certainly is possible for the US to focus on Canada proper w/o dealing with the HBC/NWC lands, w/o the Red River Colony, and w/o the Maritime then-colonies; it's basically the same MO of the War of 1812. Even then, though, like last time much of the focus will still be Canada West/Upper Canada (what we now call Ontario in modern times), and the reasons are basically rooted in racism and a changing context. By ~1860 or so you already have the French-Canadian migrations into New England, Upstate New York, and ultimately even the Midwest and what we now call Western Canada (and even then, in the case of *Western Canada, it was just basically augmenting existing French-Canadian communities, i.e. in *Manitoba), and even then early on this migration was opposed by both the clerical nationalists and even New Englanders themselves. In New England, either it's another wave of The Papist Invasions (TM/MC) or - if one was, say, Irish - a wave of scabs who don't even bother speaking English (or so they thought) taking away all the jobs in the mills that they themselves wanted for much less pay (in other words, French-Canadians were scapegoated as essentially the Mexicans of 19th century Evangelical Protestant New England). North of the border, the clerical nationalists opposed this new mass migration because they wanted to keep as many French-Canadians inside Canada East/Lower Canada as possible, even if it means colonizing marginal lands inside the province, in order to fulfill their beliefs that God had ordained French-Canadians to only be farmers - even though it was the mid-19th century agricultural crisis which the nationalists had not yet provided a solution for that was causing the migration. As a result, with some minor rumblings aside, Canada East would be preoccupied with other matters and thus would probably (not yet) would take any chances of making a fling with the US onto the back-burner - nor, for that matter, would New Englanders be willing to fight for the US if it means taking on more Catholics into their region. (Such was the unfortunate attitudes of the times.) So even with a Civil War-type effort Canada East would fall out of the US's grasp, and hence Canada West would be the main target. In that case, Britain can defend it all it wants but I could see situations both ways, either with Britain retaining it (making it a futile effort for the US) or the US conquering it (taking advantage of British military weakness) and putting down any rebellions that result from it. Fool's gold, basically.

Then, if the US does take control of Canada West, then the Province of Canada is just limited to what would used to be called Canada East, and could spook enough people in the Maritimes, Newfoundland (if the West Coast manages to get its voice heard over the sectarianism going on in the east of the Island) and *Canada to link up into a federation - as long as, because *Canada would be too dominant as one unit, the remainder of the Province of Canada got split up into at 2-3 provinces (one centered around Montréal, one centered around Québec City, and maybe one centered around Trois-Rivières, which could otherwise also be rolled into the Québec City-centered province). All that would be left for territorial expansion in this case for the United Kingdom of Canada, Acadia, and Newfoundland (a potential suggestion on my end for a possible long-form name for this TTL union) would be northwards (including the British Arctic Territories), unless if the Red River Colony can be persuaded to join up with Québec City, and hence the Province of Assiniboia (the TTL version of OTL Southern Manitoba) would be an exclave of *Canada.

 

[HB of CJ]

If a non divided USA put an all out Civil War level attack through Canada there is not much the British could do initially. Given precious time the USA Navy and Army would sweep aside any English response. The question is could the USA KEEP Canada. If it came down to industrial potential, England would lose. The problem would be keeping the war regional. It would quickly expand to a world wide conflict between the USA and England. Thus the rub. Outcome? Long term? the USA wins but at a terrible cost. Just me.

 

[stevej713]

Most people who discuss this topic overestimate the value Canada was to Britain at the time. While Canada today is a highly developed and wealthy country, for much of the 1800s it was a backwater. I personally don't think the UK would have devoted much resources to the defense of Canada. For the West Indies? Absolutely. But Canada in and of itself did not contribute much to the Empire. At varying points in the 1800s, numerous British political figures more or less expected that it would some day naturally fall into America's orbit. I'd argue the US believed the same, hence war was perceived as unnecessary.

 

[Mitchell Hundred]

Most people who discuss this topic overestimate the value Canada was to Britain at the time. While Canada today is a highly developed and wealthy country, for much of the 1800s it was a backwater. I personally don't think the UK would have devoted much resources to the defense of Canada. For the West Indies? Absolutely. But Canada in and of itself did not contribute much to the Empire. At varying points in the 1800s, numerous British political figures more or less expected that it would some day naturally fall into America's orbit. I'd argue the US believed the same, hence war was perceived as unnecessary.

Wasn't Canada also viewed as a way to limit the growing dominance of the US? To not let them take over almost the entirety of the North American continent, it would have been not even 20 years earlier the US absorbed the majority of the Mexican territory on the continent.

 

[Dan1988]

Wasn't Canada also viewed as a way to limit the growing dominance of the US?

Not that I'm aware of. At its core, Canada (and Nova Scotia) was basically booty from the Anglo-French Wars, hence why it's known in Canadian history (particularly in Québec history and French-Canadian history) as The Conquest. Now granted, part of Canada was hacked off to accommodate American settlers (not all of them Loyalists) who were hungry for land, but this was not out of the generosity of the British but more to limit the boundaries of French Canada, in particular the extent of the pre-Revolutionary Coutume de Paris within the re-baptized Province of Quebec to what would become Lower Canada (> Canada East > Québec). Which is important because at this stage the French-Canadian areas were the most populous of the Canadas, with Montréal as its economic center and London (through Québec City) as its political capital. Upper Canada, by contrast, took a long time to catch up and in order to do so, with no economic center at this stage (Toronto took a very long time to catch up to and eventually surpass Montréal), required a constant infusion of immigration. As a result, Canada West (< Upper Canada) was more vulnerable than Canada East (< Lower Canada) in terms of US potential for annexation.

 

[Fabius Maximus]

If a non divided USA put an all out Civil War level attack through Canada there is not much the British could do initially. Given precious time the USA Navy and Army would sweep aside any English response. The question is could the USA KEEP Canada. If it came down to industrial potential, England would lose. The problem would be keeping the war regional. It would quickly expand to a world wide conflict between the USA and England. Thus the rub. Outcome? Long term? the USA wins but at a terrible cost. Just me.

With what resources would the US be making this attack? The US army was only 16,000 strong, and although Britain didn't have a big army by European standards it could easily have shipped an army sufficient to defend against a small invasion at short notice. And as noted above, it took around two years IOTL for US domestic military production to reach a level where the Union Army could be supplied by mostly US-made products -- before that, the US had relied on imports from abroad, chiefly Britain, to make up the shortfall. When it comes to ramping up the armed forces for a major conflict, then, the UK has a bigger army with more relevant experience to start from, is better placed to produce large numbers of firearms and other military equipment, and has better access to world markets to make up any shortfall (as Britain's first action in the war would surely be to put the US coast under blockade). So for at least the first two or three years I expect that Britain would have the advantage when it came to putting armies in the field. Eventually the US would get to a position where it could overpower the British, but for the US to still be fighting after so long at such a disadvantage would probably require either inexplicable passivity or inexplicable incompetence on the part of the British commanders.

 

[EnglishCanuck]

Most people who discuss this topic overestimate the value Canada was to Britain at the time. While Canada today is a highly developed and wealthy country, for much of the 1800s it was a backwater. I personally don't think the UK would have devoted much resources to the defense of Canada. For the West Indies? Absolutely. But Canada in and of itself did not contribute much to the Empire. At varying points in the 1800s, numerous British political figures more or less expected that it would some day naturally fall into America's orbit. I'd argue the US believed the same, hence war was perceived as unnecessary.

We're really not. We know exactly how much stock Britain placed in Canada (and any part of her Empire really) as when Seward made a comment that Britain would not defend it during the Prince of Wales visit in 1860 the Duke of Newcastle said "Do not labor long under such an illusion... touch us once upon our sacred honor and we shall bring the bricks of New York and Boston raining down upon your heads" (paraphrasing).

There was no chance Britain would have allowed Canada to be annexed by force. They would have allowed Canada or the provinces to leave willingly if that was their choice, but if they were attacked Britain was basically honor bound to defend it. Many in the US did (erroneously) believe that Canada would choose to join the Union, but Canadians were very much disinterested and had three times in less than 100 years taken up arms to not become part of it or repel republican insurgents. Britain wasn't going to just turn it out since that would look bad to the other great powers.

Not that I'm aware of. At its core, Canada (and Nova Scotia) was basically booty from the Anglo-French Wars, hence why it's known in Canadian history (particularly in Québec history and French-Canadian history) as The Conquest. Now granted, part of Canada was hacked off to accommodate American settlers (not all of them Loyalists) who were hungry for land, but this was not out of the generosity of the British but more to limit the boundaries of French Canada, in particular the extent of the pre-Revolutionary Coutume de Paris within the re-baptized Province of Quebec to what would become Lower Canada (> Canada East > Québec). Which is important because at this stage the French-Canadian areas were the most populous of the Canadas, with Montréal as its economic center and London (through Québec City) as its political capital. Upper Canada, by contrast, took a long time to catch up and in order to do so, with no economic center at this stage (Toronto took a very long time to catch up to and eventually surpass Montréal), required a constant infusion of immigration. As a result, Canada West (< Upper Canada) was more vulnerable than Canada East (< Lower Canada) in terms of US potential for annexation.

In 1861 the Canadian population was:

Canada East - 1,111,566 (roughly 840,000 French speakers)

Canada West - 1,396,091

Nova Scotia - 330,857

New Brunswick - 252,047

Prince Edward Island - 80,857

In terms of economics, while Montreal was the largest and most cosmopolitan city, Canada East was still primarily rural and lacking in industry. The vast majority of the factories, forges, rolling mills, and other manufacturing plants existed in Canada West. Montreal and Quebec had the monopoly on ship building and trade, but certainly did not eclipse all of Canada West in productivity. Toronto had a population equal to that of Quebec. IIRC Canada West also payed nearly three quarters of the taxes for the Province of Canada. The English population far outnumbered the French speaking one, which was seen in the fact that in the West the people argued for 'Representation by Population' (or Rep by Pop) which was anathema to the French since they were badly outnumbered, and it was one of the major voting issues of the day before the Confederation debates.

Geographically Canada West was more at risk from invasion (from the nature of the frontier and border) but it was not less than Canada East.

 

[Thane of Fife]

(as Britain's first action in the war would surely be to put the US coast under blockade).

Is this really that plausible? I'm not an expert on the blockades or navies of the time, but a perusal of wikipedia suggests that the US had hundreds of ships on blockade duty, and there were still a lot of ships that got through. The British navy is bigger, but

a) The British will have to blockade more coastline than the US did.
b) The US did not really have to contend with a Confederate navy, whereas the British will need to either destroy or contain the US Navy.
c) The British will almost certainly be needing to move troops and supplies from Britain or other parts of the empire to North America on a scale that the United States would not really have needed to. Civilian ships can probably help with this, but they will probably still need some degree of navy support.
d) The British have a global empire that will presumably require some naval presence elsewhere.

This is not to say that the British can't build up to the point of being able to institute such a blockade, but will they really be able to do it from the earliest moments of the war?

 

[galanx]

"Not one man in a thousand cares whether the Canadians prosper or fail to prosper. They care that Canada should not go to the States, because,—though they don't love the Canadians, they do hate the Americans. "- Phineas Finn; Anthony Trollope, 1867

 

[marathag]

Royal Navy wore out more ships blockading the USA during the Revolution, since there wasn't much of a Continental Navy, and the more effective Privateers generally avoided contact with any RN ships, if possible.

During the War of 1812, Privateers were very successful in increasing what Lloyds charged for insurance, and captured 1200+ vessels, from 625 vessels that were issued Letters of Marque, despite a Convoy system in place

There were a lot more US flagged vessels that would give that a try in 1860

 

[Mitchell Hundred]

"Not one man in a thousand cares whether the Canadians prosper or fail to prosper. They care that Canada should not go to the States, because,—though they don't love the Canadians, they do hate the Americans. "- Phineas Finn; Anthony Trollope, 1867

Which goes to my point that the British viewed Canada at the least as a buffer to US expansion and would defend it as such.

 

[RodentRevolution]

Is this really that plausible? I'm not an expert on the blockades or navies of the time, but a perusal of wikipedia suggests that the US had hundreds of ships on blockade duty, and there were still a lot of ships that got through. The British navy is bigger, but

a) The British will have to blockade more coastline than the US did.
b) The US did not really have to contend with a Confederate navy, whereas the British will need to either destroy or contain the US Navy.
c) The British will almost certainly be needing to move troops and supplies from Britain or other parts of the empire to North America on a scale that the United States would not really have needed to. Civilian ships can probably help with this, but they will probably still need some degree of navy support.
d) The British have a global empire that will presumably require some naval presence elsewhere.

This is not to say that the British can't build up to the point of being able to institute such a blockade, but will they really be able to do it from the earliest moments of the war?

a)yes but the key point is that they in fact only need to interdict a certain number of ports. In addition British bases are not merely arrayed handily around the US East and Gulf Coasts but also across every trade route globally...especially the most important ones from Europe.
b) The British have far more and far stronger ships than the US including as well as steam driven ships of the line (of which the USN is lacking), steam driven frigates also steam driven armoured floating batteries
c) Conversely the US will be moving men and material by land which is anything from 10-100 times more energy intensive than sea travel and in addition will need to import large amounts of gunpowder and modern weapons if they are to face British reaction forces, the British had some 70,000 troops free to deploy in the home area backed by around 25,000 in depot companies and in addition had a scheme in the event of a major war to take up 22,000 men from the Militia to free regular troops from garrisons in the Empire. The Royal Marine Light Infantry and Royal Marine Artillery between them mustered 16,000 in 118 divisional companies and 17 artillery companies. Were the US to be starting from roughly the same starting position as ACW you might want to examine the lack of training provided to most of the State Militia and Volunteers as evidenced by Bull Run.

Pre-Civil War US weapons stocks consisted mostly of smooth bore weapons and most cannon were of a size suited to coastal defence or fortress emplacement not mobile use. The British on the other hand were introducing the Armstrong breech loader and had large numbers of bronze 9 pounders for field service. The Regulars, Royal Marines, Royal Navy personnel on landed service and by this stage the Militia and Volunteer Force were armed with the Enfield Rifle-Musket and mission specific variants and a surplus towards providing for war usage was being assembled.

The British had possibly the largest merchant fleet in the world, not to mention ample funds to hire foreign shipping which would be unlikely to be required. Further a much greater proportion of the British fleet had steam as either auxiliary or primary propulsion while the US merchant fleet while of similar size IIRC at this time was still overwhelmingly wooden hulled sailing ships.

d) The Royal Navy had approaching 400 ships in commission most of which were in ordinary, once the Royal Navy mobilised for war there would likely be ample ships for the Empire and home defence and away matches in the Atlantic and Pacific.

As to blockade; a blockade is a formal notification to neutrals. To be legal it has to be effective. Effective suggests that a reasonable number of ships attempting to breach the blockade could be intercepted. The US blockade of ACW was able to work from early on because the British decided it was effective and pretty much everyone else in the 19th Century followed the rule of thumb that a blockade was defined by the Royal Navy. In addition a blockade need not be declared against a coast but against individual ports. This unlikely to be necessary for the RN but is an option.

Further upon declaration of war the navies of both sides are free to attack, seize and burn enemy flagged shipping anywhere on the high seas or in belligerent waters. Even without a blockade the British have the greater ability to interdict the US merchant fleet than vice versa and the British had carried a significant proportion of US cargoes pre-war (they also did in the Civil War).

Now there are issues as the POD is so much earlier than the event under discussion and has a lot of potential oddball effects but given a pre-ACW US from OTL as a baseline and a pre-ACW British Empire from OTL as similar baseline the US would be in a lot of trouble without significant pre-war preparation. It should also be noted that Britain and the various Canadian colonies combined had a roughly equal population to the US and the British had roughly three times the industrial output by most measures and even greater advantages in ship building and armaments manufactures.

 

[Thane of Fife]

a)yes but the key point is that they in fact only need to interdict a certain number of ports. In addition British bases are not merely arrayed handily around the US East and Gulf Coasts but also across every trade route globally...especially the most important ones from Europe

Yes, but only needing to interdict certain ports would have been true of the Union navy as well. More coast is going to mean more ports.

British ports around the world will no doubt be useful if interdicting US trade elsewhere, but that will have little impact on their ability to actually blockade the US coast (except inasmuch as it offers an additional deterrent to setting out at all). If anything, while these bases will no doubt be a net plus to the British war effort, they will be an obstruction to blockade efforts in that they will require some number of ships to be kept at these bases to do said interdicting.

b) The British have far more and far stronger ships than the US including as well as steam driven ships of the line (of which the USN is lacking), steam driven frigates also steam driven armoured floating batteries

Outside of perhaps a few ironclads, I don't think the advantage in quality was so serious that the British could disperse their navy over thousands of miles of coastline without making some effort to contain or destroy the US Navy to avoid being defeated in detail. And any effort to contain it will tie up a lot of ships - I would expect the British to want at least a 3:2 advantage or so.

This is not to say that the US Navy could defeat the RN and take control of the seas or that the British would be unwilling to enter battle without overwhelming odds. But any plan to blockade the US coast must account for the USN, and because the fate of the blockade will to some extent hinge on the fate of the USN, it would be foolish for the British to not want an overwhelming advantage there.

c) Conversely the US will be moving men and material by land which is anything from 10-100 times more energy intensive than sea travel and in addition will need to import large amounts of gunpowder and modern weapons if they are to face British reaction forces, the British had some 70,000 troops free to deploy in the home area backed by around 25,000 in depot companies and in addition had a scheme in the event of a major war to take up 22,000 men from the Militia to free regular troops from garrisons in the Empire. The Royal Marine Light Infantry and Royal Marine Artillery between them mustered 16,000 in 118 divisional companies and 17 artillery companies. Were the US to be starting from roughly the same starting position as ACW you might want to examine the lack of training provided to most of the State Militia and Volunteers as evidenced by Bull Run.

Pre-Civil War US weapons stocks consisted mostly of smooth bore weapons and most cannon were of a size suited to coastal defence or fortress emplacement not mobile use. The British on the other hand were introducing the Armstrong breech loader and had large numbers of bronze 9 pounders for field service. The Regulars, Royal Marines, Royal Navy personnel on landed service and by this stage the Militia and Volunteer Force were armed with the Enfield Rifle-Musket and mission specific variants and a surplus towards providing for war usage was being assembled.

This is all irrelevant to the ability of the British to blockade the US coast.

The British had possibly the largest merchant fleet in the world, not to mention ample funds to hire foreign shipping which would be unlikely to be required. Further a much greater proportion of the British fleet had steam as either auxiliary or primary propulsion while the US merchant fleet while of similar size IIRC at this time was still overwhelmingly wooden hulled sailing ships.

Canada is not going to be able to stand alone in this conflict. British population, British industry, and to some extent the British military are all overseas. These things will need to be shipped to Canada (well, the products of British industry will). I don't know who moves these things - I assume that it is some combination of the merchant fleet and the navy (even if the latter is only there in small numbers as escorts). Assuming that there is navy involvement, this will be some number of warships that are not able to be on blockade duty.

d) The Royal Navy had approaching 400 ships in commission most of which were in ordinary, once the Royal Navy mobilised for war there would likely be ample ships for the Empire and home defence and away matches in the Atlantic and Pacific.

As to blockade; a blockade is a formal notification to neutrals. To be legal it has to be effective. Effective suggests that a reasonable number of ships attempting to breach the blockade could be intercepted. The US blockade of ACW was able to work from early on because the British decided it was effective and pretty much everyone else in the 19th Century followed the rule of thumb that a blockade was defined by the Royal Navy. In addition a blockade need not be declared against a coast but against individual ports. This unlikely to be necessary for the RN but is an option.

[Bolding is mine] I don't believe that this is true in this case. The Paris Declaration is explicit in that it is only binding between signatories, of which the United States was not one. The US and the British Empire may agree to abide by it regardless (the US said that they would during the ACW regardless of not having signed), but that is not necessarily going to be the case.

The British may very well blockade individual ports, but my point is that I'm not sure that the British could effectively blockade the entire US coast, so that doesn't really conflict with my argument.

You say 400 ships in commission. That agrees with the number I see here, so let's go with that. There's about 70 ships (those on harbor service, those under building, and the screw guardships that perhaps aren't useful for blockade service. So 330. Let's say that 20-25% of those ships on active duty are going to be kept abroad (either because they're busy there or for attacking US commerce overseas) - excluding the screw guardships, that brings us down to something like 300 ships in total. Wikipedia says that the Union had 160 ships blockading the south in early 1861, so let's go with that, and I frequently see people throw around the number of 60 ships to blockade the Union. I don't know if that counts the Pacific coast, but let's say it does. So that's 220 ships. It looks like the USN had about 80 warships or so in 1860, so let's say that the RN devotes another 80 ships to containing them. At this point, we have accounted for basically every ship in the RN. We didn't run out of ships, so maybe they can do it. But we're cutting it really close.

Further upon declaration of war the navies of both sides are free to attack, seize and burn enemy flagged shipping anywhere on the high seas or in belligerent waters. Even without a blockade the British have the greater ability to interdict the US merchant fleet than vice versa and the British had carried a significant proportion of US cargoes pre-war (they also did in the Civil War).

Now there are issues as the POD is so much earlier than the event under discussion and has a lot of potential oddball effects but given a pre-ACW US from OTL as a baseline and a pre-ACW British Empire from OTL as similar baseline the US would be in a lot of trouble without significant pre-war preparation. It should also be noted that Britain and the various Canadian colonies combined had a roughly equal population to the US and the British had roughly three times the industrial output by most measures and even greater advantages in ship building and armaments manufactures.

Please don't take me as arguing that the United States is going to come out on top in the conflict (or that they won't). The British certainly have a lot of advantages in military industry and overseas commerce. But I think that just assuming that the United States will be under blockade is risky - that's a big job, even for the RN.

 

[The Professor]

In order for the US to objectively win at this point it would require the UK and Canada to not notice that the US is building up. That plus it would surely be a pyrrhic victory makes me wonder why?
For all we claim politicians are a shortsighted bunch we forget that they are often selfserving too. The loss to themselves this war would bring before its outcome is even certain requires a rather large idiot ball.