From what I've been able to find from historians who've engaged with this what if (including a former CoS of the British military) the specialist equipment losses would have been crippling for years. As it was Eisenhower lost his mind and thought about cancelling the invasion when the Germans managed to ambush LSTs at night and sink two (two more damaged) that were practicing landings on the British coast:
en.wikipedia.org
Two ships due to how little equipment was available to the invasion.
If they lose 10 times that plus 75% of their airborne divisions and a huge chunk of the air transport fleet it will be at a minimum a year before they could try again due to how costly the equipment was. Politically it would be suicide to increase rationing on civilians at home in the US, so no 'digging deeper' before an election especially when unconditional surrender was not a particularly popular policy before D-Day.
Aircrews were told not to stop and they faced heavy AAA fire IOTL as well as low cloud cover and despite the massive screw up with the drop still kept on with it:
en.wikipedia.org
So they'd have to abort early in the operation or deal with the consequences. If the order is given to go ahead they couldn't stop given that the landings were about to happen and their success without the paras was arguably doubtful and there wouldn't be awareness of drop conditions before they were over target. Plus as it was despite the awful situation IOTL the drop went ahead anyway and resulted in pretty heavy casualties by Allied standards.
The storm on the 5th, from what I can find, started getting worse after the invasion would have been underway given the timetable on June 6th historically. So by the time they had begun things mission inertia would have forced it ahead until it was too late to avoid the damage. The paradrops were at night between midnight and 2:30 am, which was when things were bad enough, but it only got worse from there. So more likely than not the jump goes in with all the consequences of that and the landings are attempted, but they don't make it to the beaches due to the gale force winds and choppy seas.
Manpower isn't the issue specifically it is the expensive specialist equipment and troops who would be lost. The loss of LSTs and other landing craft would be a disaster since it took years to build up and was competing with similar needs in the Pacific. Politically too it would be impossible to deprive the Pacific of resources to make good losses in Europe; the war in Europe was never as popular as the war against Japan and McArthur (among many others) was (were) extremely influential politically...among the public if not the government. In an election year there is simply no way to risk pissing off the public for fear of getting Dewey elected and the generals/admirals knew that and had no issue leveraging it against politicians. See McArthur vs. Truman in Korea. With a disaster at D-Day FDR's political standing would be at an all time low, which means leverage is not in his hands if it came down to forcing a change in grand strategy.
Again the issue isn't the weather it is the loss of expensive specialist equipment.
British casualties would have been irreplaceable given that they had already broken up divisions to keep the others up to strength by this point.
Really the only option on the table in that case is to double down on Italy as it is the successful front to that point. No way they try another Anzio though, especially after another failed amphibious operation.
I have no doubt that Churchill would want a Balkan operation, but the US didn't (not just politicians) because they viewed it as another Italy, but with even worse terrain and logistics for breaking out.
AFAIK Stalin did not want the Wallies mucking around in his 'backyard', as Russian foreign policy had always been focused on expanding their realm of influence into the region. Turkey and access to the Mediterranean had been an overriding goal. If the choice is Wallied control over the Balkans and little action outside of Italy Stalin seems to be the type who would have preferred no Wallied action in the Balkans, especially if Bulgarian and Turkey then end up outside his control. Stalin did not trust Churchill on this yet, he only made deals with him over Greece due to Soviet control over the important bits of the Balkans already. As far as Stalin is concerned the Balkans under Hitler are probably preferable to the Allies, as the German would have had a weaker hold and Stalin could undermine Nazi control over the region more easily.
Add in the failed Aegean operation by the Brits in late 1943 and that looks like a dead end as well.
Tito though probably gets a lot more resources.
Were it not an election year I'd agree with your assessment. FDR was not as invulnerable as people like to pretend, it was the successful D-Day, liberation of Paris, and Pacific success that guaranteed his blow out in November IOTL. With this sort of disaster his Europe first and Unconditional Surrender policies are going to be an electoral albatross. In fact, going back to your point about pillaging the Pacific for resources, politically FDR might have to prioritize the Pacific just to have a shot at winning in November, so he could show some success. I'm not arguing that the US would up and quit the war, just that FDR would be under a lot of pressure to drop unconditional surrender as a fixed policy and would have the public backlash leading up to his historical fourth run for president as his health started seriously declining. It might even be possible that the Democratic party leadership refuses to let him run again.
Butterflies from this POD for the US domestic situation are pretty numerous, which is why I wanted to focus primarily on the US domestic situation in this thread, as the topic is quite interesting from an alternate history perspective.
Not necessarily as much as you'd think. Roughly half of the divisions inn the west would be redeployable east at some point soon, with 4 panzer divisions immediately redeployable and available before June 22nd, when Operation Bagration would happen. Of those 4, 2 were extremely well equipped, trained, and oversized relative to normal full strength panzer divisions.
Those would be the 12th SS and Panzer Lehr divisions. The other two would be the 21st Panzer division, though that one is unlikely to leave France given it's French equipment and anti-invasion training, and 2nd panzer division, a nearly elite division given its long existence and lots of combat experience. It had been rebuilt and was ready to go in June 1944.
So 3 of the divisions were likely to arrive in the East and be operational around June 21st assuming as of June 7th they all depart from France by express train, as they similar transport after Bagration only took 10-14 days.
Then there were all the other panzer divisions in the west of which there were 6 which were being rebuilt or built for the first time and were getting close to being deployable. IIRC they were 9th, 11th, 116th, 1st SS, 2nd SS, and 17th SS. That's not counting other divisions that were rebuilding which were not available for Normandy like the 6th, 19th, 5th SS, which showed up after Bagration in July. I'm not counting the HG parachute panzer division in Italy, which also did show up there IOTL, but might not here due to the large number of reserves available from France.
Then there are the infantry divisions (including 3 airborne divisions in the II parachute corps and 1 air landing division) and additional brigades like the Tiger and StuGs. There was the equivalent of another Eastern Front army group available to redeploy East if there is no threat to France for the coming 12 months. Also II SS Panzer corps stays in the East too, which is a huge gain, not just from the divisions, but the entire corps support/firepower apparatus on top of the divisions.
Even assuming Bagration succeeds, which is questionable if they have 3 extra panzer divisions ready before Bagration started of which two were extra large and powerful elite ones, there is still enough divisions capable of redeploying in June-August to replace it entirely as well as the replacements not needed in the west which can refill damaged divisions instead. There were about 2200 AFVs committed to Normandy during June-August 1944, >90% in June. Likely German positions are shoved quite far back, but the 3 panzer divisions could prevent the pockets at Minsk and Bobyirusk being closed, which means roughly 60% of the troops/equipment lost in Bagration could be saved.
IOTL only a single full strength panzer division was available and it was a regular sized panzer division, not a extra large elite one like the 12th SS (about 55% larger than a standard army division with lots more firepower that the regular army didn't get while it would also have access to an attached SS Tiger battalion), and a half strength regular army panzer division from army group north, which somehow still managed to rescue something like 50,000 men from the Bobyruisk pocket when they unexpectedly showed up and attacked while the pocket was still being formed. Both the panzer divisions that were send IOTL arrive days later than the Panzer divisions I'm talking about would have when it was already too late to save the majority of troops caught in the pincers.
If 12th SS and Panzer Lehr are sent to confront the Soviet 5th Tank Army they'd be able to stop it cold, especially if committed early in to the battle before the 78th Sturm division is overrun (it actually checked the Soviet advanced in its sector by itself tanks to the strong 2nd line defensive position it held and the massive amounts of firepower attached to it until a cavalry corps managed to move around its flank through a swamp and no further reserves existed to check them. This opened the way for the 5th tank corps to exploit to Minsk). Similarly if the 2nd panzer division is available to counterattack at Bobyruisk earlier than 12th panzer did they'd have kept an escape route open for a much larger number of troops to escape given that it was twice the size of 12th panzer by that point. If reinforced with infantry division they might even check the Soviets quite a bit further east.
Since those three panzer divisions + some number of infantry divisions would be used to support Army Group Center then AG-North and North Ukraine don't have to send troops to help them instead. So when the Lwow-Sandomierz offensive starts they have an extra panzer division and a number of extra infantry divisions (IIRC after Bagration they sent 8 divisions total to Belarus) plus the II SS Panzer Corps not sent to France since there is no Normandy invasion for them to deal with. 10 extra divisions that were missing IOTL. Since that operation was nearly defeated as it was IOTL thanks to very strong defensive positions and counterattack forces well deployed (other than 8th Panzer which somehow got lost during a counterattack and then blundering into a Soviet air attack which compromised their defensive plan when it still had a chance of working), having extra troops solves the problems they had IOTL.
Without the defeat in North Ukraine then AG-South Ukraine doesn't get stripped of the ~7 panzer divisions that helped it crush the Soviet offensive against Romania in April 1944 and the loss of which allowed the Soviet offensive in August to crush Axis forces in Bessarabia and convince Romania to switch sides. So that position can hold and Romania is still in the Axis camp.
Army Group North was able to hold on the Panther Line indefinitely, so should be able to continue to hold so long as their flank isn't turned if Bagration doesn't succeed nearly as much as IOTL. Plus they could keep the 12th panzer division, which even if only half strength still represented a significant mobile reserve. If they hold their position than Finland doesn't get knocked out either, as the loss of the Baltic states in the aftermath of Bagration (July-September) effectively put the nail in the coffin of the Finns, since it meant the Soviets dominate the Gulf of Finland and could start using their navy in the Baltic once again. In fact they did and started using submarines to sink Axis transport/supply shipping in the Baltic sea, which had consequences for trade with Sweden and supply runs as well.
So while Axis forces would still suffer significant defeats in Belarus they'd have more than enough reserves to replace them AND check the other Soviet offensives of Summer 1944. That means the line actually holds other than a shorter withdrawal than IOTL in Belarus. IOTL AG-Center if possible was to fall back on the Beaver Line, which was a prepared defensive position based on a major river east of Minsk. Assuming the Panzer divisions go to AG-Center, which they should given that AG-Center was pillaged of theirs to reinforce AG-North Ukraine in April-June, then Bagration is nowhere near the success it was IOTL and though they probably inflict around 150,000-200,000 casualties on the Germans the Soviets don't wipe out entire armies like they did IOTL, nor do they capture Minsk, so that even with equipment losses AG-Center, thanks to reinforcements from France are able to hold the very strong defensive line east of Minsk. In turn that means AG-North holds and the entire line in the East is able to be maintained through at least Summer 1944 with the Soviets taking very heavy casualties, but not being able to inflict the heavy ones they did historically without a breakthroughs that enabled the smashing of entire Axis armies. Nor do they flip the Axis allied states like Bulgaria and Romania and gain those troops.
Meanwhile the Germans don't lose 400,000 men in France in June-August either; some would be lost in the East when sent as reinforcements (or stayed since they weren't transferred to France like the 9th and 10th SS divisions) of course, but since they are 'extra' compared to OTL that is a net gain. The Soviets are in a bad position if they don't advance and gain all the manpower they did historically in the territory captured from June 22nd and onwards as well as new allies and knocking Finland out of the war allowing for extra reserves to be sent to Poland. So a vicious cycle starts for them rather than the Germans. That doesn't mean the Axis isn't worn down in the East, but relative to OTL they actually 'gain' manpower given that their losses would be so much lower than they were historically in 1944, while Soviet ones would be worse without France as a distraction for German strategic reserves.
I wouldn't be so sure. Britain did have the ability to make a deal and ask the US to leave, but that is very unlikely even with a disaster on D-Day. However if the V-weapons problem breaks public morale since there wouldn't be a way to overrun the launch sites on land thanks to the invasion forces, that could potentially be enough.
I'm not so sure that the average person in Britain cared as much about geopolitics as the ruling class did and if enough average people demand an negotiated peace deal they could force the ruling class to comply. Strikes would shut down the ability to wage war if enough people got pissed off enough about the course of the war. Churchill and the government were pretty terrified IOTL as of 1944 due to the V-weapons and army casualties of the public demanding a negotiation over total victory.
The V-weapons were only defeated by the land invasion shutting down the launch sites; even with the land invasion there was no way to actually stop the launches until the sites were overrun. Op Crossbow failed despite extremely heavy bombing of the launch sites.
en.wikipedia.org
In fact they were considered by the Allies as a massive gain for Hitler, because 70,000 bomber sorties were diverted from attacking Germany or military targets to bomb the launch sites which were not shut down, just inconvenienced. They'd have to dedicate the bulk of Bomber Command to hitting the infrastructure behind the sites (and piss off the French against the Allies) to even have a small chance of stopping the attacks. What happens when Germany then is able to avoid the worst of the 1944 bombing due to diversion of attacks?
So instead of increasing bombing of Germany instead the Allied bombers are locked down wrecking France to try and stop the V-weapons. How long before Frenchmen opt to start fighting with Germany rather than against it in that case? A lot of French were furious about the OTL bombing:
en.wikipedia.org
Coupled with the failed invasion French public opinion could turn quite hard against the Allies.
Before you say it was connected to the Normandy campaign, much of the bombing was pre-D Day too:
en.wikipedia.org
Add in more Crossbow bombing and the bombing of France is going to continue and escalate without the ground campaign, but it will be concentrated against any area where the V-weapons are located. How the French react to ceaseless bombing by the Allies is up for debate, but the indication wasn't that they'd turn on the Germans, since they didn't really do that until after the invasion was a success and the Germans were being beaten, so resistance was thought to be more politically acceptable.
Antony Beevor's Normandy book cites a number of instances where French civilians attacked Allied troops in Normandy because they were so furious about the bombing.
Which is why I laid out the military situation in the OP so we could focus on the political/social impacts.
Probably success of what operations?
I'm interested in alternate history scenarios and WW2 is one the most well studied and impactful times of the 20th century; if you're not interested in exploring alternate worlds these sorts of thread may not be for you.
As to what people of the time knew/understood I think you don't actually understand what people of the time actually thought, let alone would have been able to achieve given the situation. The success of the Allied forces IOTL was extremely hard won even after D-Day succeeded, so even that late things were not a foregone conclusion and there was a lot of concern among top leadership, especially Churchill, of what the consequences would have been had the invasion failed. For good reason, since he actually understood what it would have meant for the alliance and war effort.