Remember air forces and navies are equipment heavy, rather than manpower. Whether one or both sides would take advantage of the changes or not is an open question. Do the Germans become complacent or the shocks in the east are enough to go to a full war economy that does not take damage from air attack and becomes more efficient than historical? If there are almost no day sorties being flown over Germany does that mean the Luftwaffe makes more effort to shoot down the reconnaissance machines?
An offensive air or submarine doctrine sets you up for many casualties, the operations are deep enough in enemy areas damage is much more likely to be fatal, the best to usually hope for is crews are captured. The U-boat operations were the big German offensive, for much of the war the Luftwaffe stayed over friendly territory or made shallow penetrations into enemy airspace.
Do the allies use the freed airpower for anti submarine operations, escorting convoys as far out as possible plus mounting barrier patrols around the U-boat bases? Does the navy respond so the better mid war escort tactics come in earlier? Without the bomber war do the better radars and navigation aids turn up sooner or later? The worry about radio transmissions made it hard for aircraft and convoy to find each other, made easier when the aircraft had radar. A big one is whether air cover is put in place in the western Atlantic and Caribbean in early 1942. Saving merchant ships in the 1939 to 1942 period is a good way to shorten the war.
On 21 June 1941 the Luftwaffe night fighter strength was 263 aircraft, still around that a year later but ending 1942 at over 400, reaching 585 end June 1943, that force is mostly available for redeployment. The day campaign takes until sometime in 1943 before it has an effect on Luftwaffe day fighter strength and deployments, the force peaked at 1,477 on 1 June 1940, rebuilt to that level a year later, run down again, to under 1,400 in mid 1942, built up again to 1,514 end October, passed 1,600 in May 1943, 1,849 on 30 June, run down again before the increases in 1944. The late 1943 into 1944 decision to cut bomber in favour of day fighter production would not be as severe.
Fighter altitude performance would probably remain the same, given Luftwaffe tactics, the Germans were not that worried about bombing France etc. until the allies obviously had an invasion force, the fighting in the MTO was episodic making most air fighting duelling air forces until at least end 1942 and possibly to the invasion of Italy, with an interlude around Tunisia in early 1943.
German resources freed to end 1943: most of the night fighter force from 1940, the increase in AA guns from 1940, the extra raid tracking personnel, the work to repair air raid damage, the loss of production due to damage and air raid alerts. Deployments and some of the numbers of the day fighter force in the second half of 1943. It is actually not a lot in the overall war situation and the size of the German economy and this can be directly related to how end weighted the strategic bombing was, meaning it was 1944 before it firstly hurt the Luftwaffe and secondly made obvious impacts on the German war economy. The 1943 area bombing did a lot of damage but the effects on the war economy were less, the Germans could make adjustments to effectively deflect at least some of the damage to less important areas.
In terms of sizeable amphibious operations first there was Dieppe, where the WWI lesson of strong naval gunfire support was relearned and that amphibious assaults into urban areas is combining the two hardest ground operations, do not do it. Then came Guadalcanal, which confirmed amphibious operations where you do not have command of the sea and sky are a very bad move. Next was Torch where the landings on the Atlantic coast lost some 25% of the landing craft, confirming the sort of anticipated loss rates. Then came Sicily, where some of the invaders were met with heavy weapons in the form of armoured attacks, and proved naval gunfire support was a good thing but it would have been best to ensure more army heavy weapons were ashore, equals more LSTs and LCTs (A Boise class cruiser makes a great anti tank battery, 6 inch guns, high velocity, flat trajectory, tons of fire control, crew trained to hit moving targets but really needs the enemy tanks near the beach to see them). Sicily also showed the US the standard amphibious supply technique was only geared to small forces on small landings. If an Army was coming ashore and they were going to move large distances it was imperative a proper Army logistics system was put in place, not just a large number of beachhead supply and control units. Ignoring the various landings in the Solomon Islands since they were against lightly defended beaches on isolated islands we come to Salerno. This was the first time the opponent had major mobile reserves, including tanks, and the chance to deploy them. The result was the beachhead roped off and for a time in peril of being destroyed. It could be argued it was held by the air and naval power but only freed by the threat from the arrival of ground troops from another landing, partly because there were no follow up troops. Then came Tarawa with the Dieppe like lesson of how many casualties would be taken assaulting a defended coast, especially if fire support is not available as the troops touch down and, if possible, while they try and cross the beach. Such fire support had to be delivered close inshore which meant largely from converted landing craft. Anzio once again showed how hard it was to break out of a beachhead but also how hard it was to destroy one as long as the ships could support and the invader had good control of the air. The assaults on the Marshall islands in February 1944 showed better techniques could cut the casualties for opposed landings.
From experience in the Mediterranean the allies knew beaches did not "cut up" over time, that minor ports could be given useful boosts in capacity and that DUKWs were very useful. This experience did not stop the infantrymen on Omaha being overloaded, at least 68 pounds each, but that was down from the 132 pounds per man at Torch. Also it is unlikely the D-Day landings devoted assault cargo space to athletic equipment, barracks bags and administrative records like Husky did. Other things learnt included using trucks on beaches was a bad idea thanks to effects of sand and salt water. Send lots of food (or tighter controls) since assault troops have a habit of hoarding food, at Torch 12 days supply of food was gone in less than 4 days, causing shortages, this also happened in Sicily but to a lesser extent. Also good beach and cargo control was needed otherwise cargo on the beach was either someone else's problem or unguarded cargo a golden opportunity to (over) stock up, just in case.
The invasion of Sicily had shown amphibious shipping losses were much lower than expected, well down on the 25% figure for the craft used in the Atlantic Ocean beach landings in Torch. This was good, but the down side was the shipping was needed for a much longer period afterward than planned to supply the troops, indeed some of the shipping was still being used for Sicily when the allies invaded Italy. This was bad as the landing craft in particular had not been designed for prolonged use, after all if you were going to lose so many each invasion why make them that durable? The result was a long repair bill at the end of the operation and less shipping for follow up assaults than had been expected.
The world did not have the number LSTs used in Overlord until the end of June 1943, assuming every LST made was serviceable and sent to Normandy, and it took around another 4 months to train the crews properly. The US started 1940 with a nominal 7 divisions, up to 23 by end year, 39 by end 1941, 76 by end 1942, 94 by end 1943, each new division could be considered to make at least one existing division non battle worthy by providing cadres to the new formation. Of the 76 divisions end 1942 11 were in the Pacific, up to 17 by end 1943.
As has been pointed out there were simply not the trained troops, amphibious shipping, merchant shipping and airpower to mount Overlord in 1943 and it would be against better defences, the forces historically used at Kursk, which would have more mobility than in 1944 as allied airpower is less, also less paratroop lift. Expect the allies to make the same sort of mistakes they made attacking Sicily and Salerno if those operations had not been done. The Mediterranean would still be closed.
When it comes to Japan the B-29 raids caused less damage to industry than the effects of dispersal to avoid the damage. As noted the mining campaign was quite effective along with wrecking the inter island ferry system. According to the USSBS Japan lost 210 merchant ships to mines, of which 25 were to end 1944, another 16 in Q1/45 then by month April to August 1945 it was 16, 66, 45, 34, 8. Also from April 1945 another 143 ships damaged but not repaired by the surrender.
Early figures for China suggested about 10 million dead but later figures indicate twice that, which works out to around 100,000 to 200,000 deaths per month on average. Trouble is there was conflict before and after the Japanese invasion. Many civilians starved thanks to things like the rice offensives where the Japanese advanced into an area just after the rice harvest, took the food and left. The lack of air raid warnings and shelters meant a high death toll despite relatively low numbers of air attacks. Indo-China had a famine in 1945 that killed 700,000 to 2,000,000 people, rice rations in Malaya in 1945 1/5 that of 1942, if people could obtain them, food deliveries to people in Manila effectively stopped in June 1944 (Guerrilla warfare played a part in this one). See the book Food Supplies and the Japanese Occupation of South East Asia
Given Japan was beginning to starve and the government was going to cut rations to women and children first many Japanese would die in addition to the deaths throughout the rest of the war zones. Historically there were starvation deaths in Japan in 1945/46. In the period mid 1946 to mid 1947 the US shipped some 800,000 tons of food aid to Japan.
Leading to the opinion pieces about what was the least human life cost way to end the Pacific War.
