… and you have perfect 20/20 hindsight. What’s your Pacific theater strategy?
Limit offensive operations and avoid invading any islands. Build up overwhelming superiority in aircraft carriers and submarines. Give highest priority to fixing problems with early-war torpedoes. Use code-breaking to hammer the Japanese if they attempt any attacks. Use submarine warfare and carrier strikes to whittle down the Japanese fleet and to starve Japan of strategic supplies. Develop nuclear weapons. Use a submarine to demonstrate a nuclear bomb in Tokyo harbour (a few miles out). Offer slightly better peace terms, including guarantee of Emperor’s safety. Hopefully would knock Japan out of the war with the fewest number of casualties on each side.
Fix the torpedoes, island-hop your way closer to the main islands, delay using nukes until we have several dozen, then pave Japan. Make Halsey a prophet.
Screw the theater, I’m goin’ to Disney World! Just as soon as it’s built. 
Lift off, and nuke the site from orbit.
[sub]It’s the only way to be sure.[/sub]
Put Alka Seltzer in charge of military operations in the PTO.
Diplomatically, I would keep the requirement for unconditional surrender. Close the Burma road and any suppport to China is shared equally between the Commies and Nationalist (with an eye toward relations with either side after the war.)
Forget the two-pronged approach from Central and Southwest Pacific. Focus solely on the Southwestern route, for three reasons:
- It concentrates your growing forces, rather than dividing them.
- It puts more economic and strategic pressure on Japan by depriving them of (or at least making it harder to get and transport) the oil, rubber and other materials they need.
- It’s the shorter route to Japan proper.
As an added bonus, many of the bloodiest battles of WWII (Iwo Jima, for example) can be eliminated.
Establish Nimitz as the supreme commander, with MacArthur in command of all ground forces. Use the fleet (particularly the carriers) as shields against Japanese air power, which was on the decline at that point anyway. Use the submarine fleet to sink the Japanese merchant marine force, particularly the oilers.
Strengthen the troop disposition in China to guard forward air bases; use them to launch B-29 raids against the Japanese home islands on a regular basis. Once the Philippines are taken, move the primary B-29 bases there.
Leave Rabaul, Truk, the Gilberts, Marianas and Marshall islands in Japanese hands, but use fleet carriers to bomb them regularly to reduce their ability to house/launch aircraft.
nothing much for the next 6 months. you need at least one more year to build up halsey’s third fleet so that it’ll be strong enough the defeat the main japanese fleet.