The invasion of Europe could not even be considered until Allied Air superiority (if not supremacy) was in hand. By 6/44 the Allies had established complete control of the airspace over the Channel and most of France. An invasion before the Allies had this advantage was unthinkable.
That's a really important point, but I think it raises a couple of others, including the most important, & often overlooked, one. First, if air supremacy is a must, a/c production might need to change. That, in turn, demands changes to Bomber Command policy (unlikely, given Harris, but not impossible in the overriding Allied interest). And that, in turn, offers an opportunity to Coastal Command to get more aircraft. (Diversions don't need to be nearly so big as wkwillis presumes: just 3-4 lousy squadrons of VLR Liberators in NF in '42!) Which brings me to the #1 issue: troop strengths. By May '43, the mooted time, did the Allies have enough manpower in Europe to successfully achieve Neptune? If not, it might be necessary to get more Coastal Command aircraft much sooner, to enable the troop buildup to succeed. This is balanced by the reduced demand for shipping to supply the troops (& civilians!) after an Italian invasion...
More to the point how will they ship them to Britain and the supplies to support them. Someone made a comment about the US shipping a lot of non-munitions to Britain. That is quite accurate. However what was shipped was the food to feed the population and raw materials for Britain's industries - which by this time meant war production. If you think that Britain can be the base for an invasion of France while the population is dying or starvation or war production has been crippled by shortages of raw materials then there's something way wrong with your understanding of the war.
True, except for one thing: AUS was getting about double the weight of supply/man as B.A., & about half of that was simply being thrown away or not used....
This ignores as mentioned that the U boats were only really defeated in the spring of 43. They could have been defeated earlier but that would have needed changes in decisions some time before mid-42. Difficult anyway to advance it much during the winter months when the weather helped the U boats.
Presuming the decision to go is made in time, the change with the biggest impact, as noted, is dead easy. (Fire Harris?

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If you think how many tons of bombs were dropped on northern and eastern France in 44 and most of that wouldn't have been possible a year earlier as less planes and less equipment would have been available.
I have to disagree. A lot could've been diverted (as OTL) from fruitless bombing of German cities.
This was also the problem with the panacea target approach.
Some of Harris' resistance was due to being denied the Ultra showing how bad the fuel situation was for Germany.
Given the right decisions and a lot of luck a lot more damage might have been done to the German economy and war machine. However we lacked a lot of the tools, such as the long ranged fighters
I agree on the first. More use of air-laid mines bombing of canals could've had serious impacts on German transportation of parts &, more important, coal to powerstations. I disagree on fighters: it was possible to execute minelaying missions with nearly zero losses; the missions were so low hazard, they were used for crew training...
Don't forget that in OTL it was only a highly efficient espionage operation persuading Hitler that the Normandy landings were a feint that stopped a lot more German troops arriving while the allies were tied up in the bridgeheads.
The XX Committee was up & running early in the war; if the decision was taken to go 5/43, the turned German agents would've put out disinformation just as they did.
Furthermore, without the intensive bombing of the German cities, supported by long ranged fighters that would not have been available earlier in suitable numbers, the Luftwaffe would have been much stronger. As Calbear said there was no way any invasion would have been risked without overwhelming control of the air.
I don't see the connection. Local air superiority was possible without bombing or LR fighters.
I also think the idea of so much less in the Med is very risky. You may not get an Italian surrender, which would not only mean Italian troops holding ground in the Balkans, but free up sizeable forces from the historical Italian front.
Fall of Sicily gets Italian surrender, just as OTL, & no mainland invasion frees more Allied troops than German, plus shipping (as noted).
True the Germans would have to cover against some landings but its difficult to see them being as much as were tied up historically, with garrisons over much of the area and a major campaign in Italy.
Hitler was convinced, with some help from the Brits (Operation Mincemeat), of an intended invasion in the Balkans.
Furthermore, I doubt the Germans could move forces out of Italy, given a hostile population... And, do the Allies give Italian POWs the option to join them invading Normandy? Also, as Grigg points out, what was the sense of crushing the Tunis pocket, rather than just choking it off? How long did that waste? 2mo?
As for the Pacific war, the earlier commitment to NW Europe means less commitment to the Pacific, however, I think it really only means means less LC's so does that mean MacArthur's drive is restricted while the USN drive through the central Pacific carries on, the Essex's etc construction programme is not going to be afftected that much if at all as the material is already going to be allocated.
I have to disagree there'd be no changes; LCs were lowered in priority when it looked like the invasion would be later, & IIRC, the
Essexes (or CVEs) bumped up (leading to a ridiculous #CVs in PTO, IMO). If it's looking like mid-'43, LCs keep higher priority. Also, with no Italian invasion after Sicily, there are more anyhow, so it's likely IMO Dragoon & Neptune go off similitaneously, as planned.
I agree MacArthur's going to get less, but that means FDR has to decide on a 1-road strategy; assuming he goes for CPac, & tells Dougie, "Sorry, you'll have to bypass P.I.", the U.S. gets to Okinawa/Iwo at least 5mo sooner (no P.I. delay), plus they're easier to take (not reinforced by troops who were destined for P.I. but didn't arrive due to Sub Force interdiction

), so take, what a month or 2 off? Bypass Peleiliu [sp?], maybe another month. B-29s begin burning Japan a couple months sooner, probably. And Japan starts looking for a surrender 5-8mo sooner, while FDR's still alive, not while HST's just taking over...
One other ish might get addressed that wasn't OTL: the wastage of shipping in PTO due to slow unloading. Hulls swung at anchor for extended periods (days; IIRC, even weeks), so the total amount of shipping used was way above what should've been needed...
Tanks & trucks don't run very well without fuel, this delayed the allies during a critical period that MIGHT have made the "Dash to Berlin" possible.
IMO, that was a product of "target fixation" on Berlin (& implicit in it, ending the war), & the squabbling between Monty & Patton, which IMO had Ike too distracted playing referee. Monty, the supposed genius, couldn't see the obvious: to
use Antwerp, the Scheldt had to be cleared, which could've been done with a couple of battalions in early September OTL. Also, in '44 OTL, IMO, there's some sense of B.A. exhaustion, from being at war so long; in '43, less so, so maybe there'd be a bit more boldness tactically, & a bit clearer vision. Maybe the breakout from Gold/Juno/Sword into Caen happens a tick sooner, maybe Monty realizes clearing the Channel ports is a futile exercise (it's not like the Germans aren't going to
blow them up), so maybe he has the wit to send Andy McNaughton fulllbore into Antwerp/Scheldt (maybe Simonds can get
him to persuade Monty?) in June/July '43 (assuming May '43 Neptune) & the dash to Germany doesn't stall for lack of fuel... (Of course, it didn't help the Red/White Ball drivers didn't have 10 tonners, 'cause planners didn't expect to need them...

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what does having Cherbourg open for business sooner do for the supply situation?
I doubt you get Cherbourg open soon enough to change much. IMO, Antwerp was the biggie, if the Allies could get there, & clear the estuary, soon enough...
I'd have to say that the prognosis that the Russians would just move west faster in this timeline seems more accurate.
It is, but for an overlooked reason: Germany used France as a training area. Take it away, reinforcements to EFront are lessened, & Red Army moves faster. Into Germany? Maybe not: May '43, Red Army was still well behind its prewar borders...
The Germans will never come close to being cauldroned as at Falaise in OTL. They will withdraw to the Scheldt and the Westwall in good order, demolishing the bridges of France along the way and doing whatever else occurs to them in the way of reducing the Allied capability to move troops and equipment around.
Why? Hitler's less of a nitwit TTL? He ordered von Kluge to attack, & von Kluge dutifully went into the trap, sprung 'cause the Allies (thanks to BP) knew he was coming... And if the Allies are doing so well early, why doesn't their success continue as OTL? In fact, why don't they do
better TTL?
Defeat of the UBoat fleet before 1943 requires earlier Enigma cracking.
Actually not. As noted, a handful of VFR squadrons in NF & Iceland'd do it. You don't need to read the contents, just DF a message near a convoy, presume it's a U-boat & figure it's liable to mean lots more where that came from pretty soon. The importance of breaking
KM Enigma is overstated.