United States really doesn't have a military In 1914 the United States Army comprised 98,000 men, of whom some 45,000 were stationed overseas. The regular army was backed up by the 27,000 troops in the National Guard. the US can draft soldiers increase their numbers but while they're doing this the British are also doing the same thing.
And the majority of those soldiers will be sitting overseas in the UK or some other Dominion/colony which isn't Canada.
any US invasion of Canada will be on a wide front so there will be no trench warfare outside of Defending cities but with the introduction of the Maxim gun defensive positions will become quite costly to take.
Why? There's nothing in most of Canada. Once you cut the Canadian Pacific Railroad then Canada is cut in two and the Prairies can be taken at will. There is no corresponding weak point in the US besides the entire Mississippi River in which case good luck. That means you have a front on the West Coast (Vancouver), the Prairies (Winnipeg), the Great Lakes (Ontario), and Northeast (Quebec/Maritimes) which isn't quite the biggest front in the world. And it really helps the US defense that there's lakes and rivers in the way (St. Lawrence River, St. John River, Rainy River, and Columbia River) the entire time except in the Prairies (the Missouri can easily be held though) where the Red River flows north and supports an attack against Winnipeg. The US rail network can support anything here far better than the Canadian one, and the CPR will be overwhelmed trying to settle troops between the east and west. I'd expect a huge buildup in Wisconsin, Minnesota, and North Dakota to both take Winnipeg and for the more immediate concern keep the British away from the copper and iron mines in Minnesota/Wisconsin/Upper Peninsula of Michigan. Which I'd expect to go well since the US will have superiority on Lake Superior and the terrain is nothing but swamps, forests, and mountains.
Ontario is not too big of a challenge when the Great Lakes will be an American lake for the duration of the war. US shipbuilding in that area stomps Canadian shipbuilding and the US has more and bigger ships on the lakes to begin with. That simplifies logistics and opens up opportunities to outflank any defense of Ontario, although one that's likely to easily result in disaster if poorly planned. American ships will be able to bombard Canadian positions and cities at will.
Same goes with the St. Lawrence River or invading Quebec since the US will have superiority on that river as soon as possible. Lake Champlain is linked to New York City by canal so small warships and supplies can be moved by boat to the Canadian border. The reverse is not true because the US can blow the Champlain Canal to prevent the British from getting through to the Hudson River. To a mch lesser extent, the Columbia and Kootenay can support (not by much since ships had to be small thanks to the rapids in that section but anything helps) an American attack into British Columbia and deny yet more resources to the British isolated here, and once New Westminster falls, the Fraser River opens up the rest of BC.
now you only have so many people willing to volunteer the rest are going to be drafted and what are they fighting for exactly it's not a defensive War you're invading Canada which is going to be well defended you do need to train your soldiers this isn't the Soviet Union you can't throw American lives away an expect the American people to support it. the United States economy has gone to crap because of the war how are you paying these soldiers can you supply them
You'd have plenty of people volunteer because the US has a huge population, including many people of Irish descent who would be very glad to stick it to Britain. What are they fighting for? Well, the British just invaded them. Obviously this would be a defensive war (or easily portrayed as a defensive war in the US) since the US isn't going to jump into a war against Britain without a huge buildup of troops, fortifications, and warships.
how would the United States respond to a naval Invasion on the west coast or south when most of their army is in the North how does the United States deal with major strikes in cities because there are going to be a lot of people unemployed because United States Navy cannot fight the British Navy in the open sea so they can't keep trade routes open with foreign Nations.
The US isn't taking an offensive stance against the British, the goal is to buy as much time as possible and keep the British fighting in the middle of nowhere in Upstate New York and the mountains and forests of northern New England to keep them away from anything useful. Any invasion on the coast will take months to prepare, which means a decent enough militia can be equipped and more coastal guns installed in addition to what's already there. That said, there's nowhere on the West Coast to attack except San Francisco, Los Angeles, and San Diego, because the majority of the coast is rocky with almost no transportation inland. The exception is the Columbia River (leading inland to Portland) which has a large sandbar outside of it that's well-known to wreck ships and the Puget Sound (leading to Seattle) which has plenty of islands and narrow passageways that would make torpedo boats rather useful.
the British have a massive Empire and the Navy that can protect it and is able to maintain trade with other nations keeping its economy going. you almost get a half a million soldiers that joined the British in ww1 from Australia and New Zealand 1.3 million from India Britain was able to supply and maintain an army of over 5 million in World War 1 this is just from the British Isles there was over 600,000 Canadians that served in World War 1 your million man army is looking kind of small if the British take this as seriously as they did ww1 the fight between Columbia and Britannia is not going to be pretty and my money's on Britannia being able to force the United States to the negotiation table.
If they take it seriously they go bankrupt because they do not have the capability to fund and equip such a large force without US finance and resources. They will lose Canada period, and the Caribbean won't last much longer after Canada falls and the US is on a war footing that makes 1941-1945 look like a joke. Because you can't invade the US and expect the US to not go on a huge war footing. A million men is just the first year, the US could and will easily have a good 5 - 6 million if needed.
also Anglo-Japanese Alliance Japanese looking at the Philippines
Which has a significant defense force which has spent many years in combat pacifying rebels. Not an easy fight (but definitely a winnable one) and a great distraction. Same with Hawaii, it already has a garrison and it will take a while to prepare an invasion for, meanwhile, Hawaii can equip a sizable force since the British won't be able to lock down the entire Pacific due to lack of bases.
tell me how is the United States making any money when its contact to the rest of the world has been cut how is the US government paying for this Army without any trade Revenue increasing the income tax which was just pushed through as a amendment to the Constitution a year ago all that would do is put more people in the unemployment line which is already rather large since the United States can't trade with other nations due to British and Japanese blockade
I highly doubt its possible to blockade the entirety of both coasts to any degree of success for more than the initial year or so. Once more American warships come online that will start coming apart and eventually start grinding down the Royal Navy and IJN. Much of the Caribbean, Central America, and of course Mexico will more or less remain open to US trade. Plus the US is a major financial center so the government shouldn't find it hard to borrow money to pay for all this.