I don't see how not sure that being allowed 70% rather than 60% of American's strength in capital ships by the WNT would have helped them.
The cruisers are what matter. Battleship combat in the Pacific War is a rare (4 times) event in the conflict, with the Americans either usually enjoying new against old, and better against worse admirals, and surprisingly (And always MISSED by relevant so called experts.) guns and anti-ship ammunition in the surface action to be immediately followed or accompanied by close air support with Samar and the Guadalcanal battles showing what combined arms means to a "gun-club" admiral who is all battleships', cruisers', destroyers' guns and torpedoes, versus an enemy who uses an air force to support his own surface forces. As the kahunas say; "Wipeout."
OTH because the Japanese were Lizzy Drippings when it came to telling the truth about their warship's displacements makes a lot of difference in the carrier battles of 1942.
That sword cuts both ways.
This is because it increases the Japanese quota from 81,000 tons to 94,000 tons. IOTL the 2 capital ship conversions consumed 53,800 tons and the Japanese used the 27,200 tons that were left to build Ryujo, Soryu and Hiryu, which the Japanese said displaced 7,100 tons, 10,050 tons and 10,050 tons respectively, which were fibs of pavarotic proportions.
-To be fair to the Japanese, they "tried" to meet their declared "treaty" tonnages, but because of bungled ship-wrighting, they had to go back and bulge and do other things to fix the bungled top-heavy float bubbles in their named construction. The actual cheating comes in at Shōkaku and Zuikaku after the LNT walkout. (and the shadow program ships.)
70% of 135,000 tons is 94,500 tons. That increases the tonnage available after the capital ship conversions were completed to 40,700 tons, which they would use to build 4 ships with an official displacement of 10,175 tons. However, the first pair, built instead of Ruyjo would be Soryu class with a real displacement of 15,900 tons and the the second pair built instead of the OTL Soryu and Hiryu would be built to the Hiryu design and displace 18,500 tons.
And were EASY to mission kill and burn down because the designs were "poor" as far as gasoline stowage, elevators, rudder and screw placement, and flammability were concerned. If the tonnage is not used well, then the IJN will find that those 4 bird-farms built to the 15,000 tonne Hiryu standard still are one bomb write-offs. This will matter with the Midway discussion in a moment.
If that was the only change between OTL and TTL between 1922 and 1940 it's likely that the 2 ships built instead of Ryujo are part of the Kido at the end of 1941 and take part in the raid on Pearl Harbour, the attack on Darwin and the Indian Ocean raid. I think they'll be refitting during the Coral Sea, but both ships will be at Midway and add another 108-126 aircraft to the Japanese air strength. I'll be prudent and say that Akagi, Kaga and Soryu are still sunk but the other 3 Japanese carriers and most of their aircrew survive. Yorktown is still sunk but the other 3 ships survive. The Japanese probably win the Battles of the Eastern Solomons and Santa Cruz.
It is likely that those 2 ships built, means one old flattop will be a training carrier to replace Hosho and the other bird farm goes north with Hosagaya to raid Dutch Harbor. So instead of Kaga, tough to burn down, and Soryu, easy to mission kill, we get 2 or 3 Hiryus and Soryu and Akagi sails with Hosagaya (old and slow). Actually that works to Spruance's favor and even Nautilus gets her chance, because she misjudged her target's speed *(a couple of fish ran ahead of Kaga as she limped ahead on fire.) and BOOM.
However, before anyone points out the obvious. I know that the Americans would have out built them by 1944 and it doesn't change the outcome of the war. However, they might not such heavy losses at the Philippine Sea if the Hiryu, ALT-Ryujo and the 4th ship survive to take part in it. If they survive that I doubt that they will be at Leyte Gulf, because there won't be the fuel or enough trained aircrew. Though having written that, they might take the place of Zuiho, Chitose and Chioyoda.
Now what do the Americans do with their carrier tonnage? Because they will use the 10,140 tonne exception in part to make up for the 70% the IJN have earned. This is a good question. But, let us do the BC subtraction thing.
135.000 tons SD
- 66,000 tons SD
...69,000 tons SD
3 x 20,000 ton SD carriers and 1 x 9,000 ton "aviation ship" ? Suppose the Americans declare their battlecruiser conversions come in at 27,000 tons? They claim they stripped out all the hull armor and are entitled to 11,000 tons SD + the 69,000 tons?
135.000 tons SD
- 54,000 tons SD
...80,000 tons SD
Ranger, Wasp, Yorktown and Enterprise are all full blooded attack carriers. Things get "sticky" as that adds about 100 aircraft to the USN seaborne aviation. Then add Hornet. So 7 against 8 theoretically in 1941. No difference at all in the actual results except that Wasp is tougher and the IJN ships, Kaga, Akagi, Soryu, Hiryu, Ryujo all still will die by fire. Then you get those 10,000 "Northampton" aviation depot and tender ships as the 10,140 tonne WNT clause exceptions. 2 x those =a Yorktown. Let us be parsimonious and say 4 get built and assigned to LANTFLT in place of Ranger and Wasp.
Coral Sea is 4 against 3 and Shōkaku does not make it, but Lexington does. Yorktown is still hit.
Midway, it is 5 against 5. Wasp with Sherman is there instead of the incompetent Mitscher and Hornet. Lexington is there with Yorktown, Fitch hands fleet air ops instead of Fletcher and then Spruance by the seat of his pants. Who jumps whom? Based on Coral Sea and with Nagumo and crew instead of the far better Hara and staff? 70 torpedo planes instead of 41? A decent fighter escort with the low boys now instead of Slim Pickens and the scrubs off Saratoga? Kido Butai's chances? ZERO. Hiryu will not survive the first Alpha. Zuikaku will be lucky if she floats home alive. Kongo will be lucky if she makes it home alive. Eastern Solomons becomes a cake walk. There will be no Santa Cruz. there might not even be a Savo Island or if it is, daylight finds Fletcher all over Mikawa.
The extra IJN ships in 1941 won't matter in the end, once the USN submarines mine intra-Japan shipping lanes.
US mines have the same magnetic influence feature problem as their torpedoes.
How soon after the atomic bombing would the invasion go in? I am thinking atomic bombs, small islands, and amphibious assaults are a bad combination for all involved. For the Americans, probably more post-war than during the war. Establishing bases on irradiated islands adds to the long-term adverse effects. Short-term, for winning the war, okay. It will be a higher butcher's bill than OTL regardless.
Truk gets a bomb right away ASAP. Rabaul earns one. I suspect that it will be more of a case of delousing and then land on the nearest flat island with no Japanese farm-boys on it and build an airstrip and then use any convenient breakwater atoll near to it to be a fleet anchorage. More Kwajalein and Ulithi than Peleliu. Still have to clean out the irradiated farm boys because some islands HAVE to be taken. Saipan and Tinian would be examples. Iwo Jima as well is an example. Once the close in for the kill happens, I suspect that bombing the home islands would be part of the anti-kamikaze campaign to protect the fleet. Okinawa would get the treatment; half and half. need some of the island for basing. It would make the Americans = to the Germans as WWII war criminals once the judgements of history roll in.
It's not just ships and planes , it's tanks, artillery and virtually the entire Western Front moving to the Pacific Front. Also you don't need the entirety of the various islands. All you need is enough land to build airstrips with a small defensive zone around them . The islands themselves are near worthless, for the most part. All you need them for is airbases.
See my above comments, JR. The thing is, the Japanese are not stupid in the op-art or when they go defense. They learn fast. They adapt and are clever in their limited choices. You see this happen in how they fight, how they plan and how they adapt. US casualties at sea and on land go way up in the last year of the war.