You can not make it from Alaska to SZ 5 in one turn with out the naval base… unless your map is different then mine. Sure, you could just take two turns to get there, but that leaves your invasion force vulnerable to both air and fleet attacks by Japan when they are stuck sitting for a turn in SZ4. It also means there is less pressure on Japan since they have an extra turn to deal with it.
The objective is not to take and hold Korea. In order for Japan to win the game, they really have to increase their income and put lots of pressure on the UK/China while controlling the DEI. In the early stages of the game, Japan has plenty of ships and aircraft to deal out a lot of damage. What they lack is the ground forces to take and hold a lot of territory and the income to rebuild the lost aircraft and ships while pumping ground units out from a factory in the mainland.
If you happen to get a foot hold in China, just drive your tanks around taking money away from Japan. Sure they can come take it back, but that requires them to divert land units from the advance on India and takes up some of the, now hard to come by income.
By setting up this move with the US, you limit Japan’s ability to heavily control the DEI since a large portion of their fleet and/or aircraft are sitting up north defending against the invasion of either Korea or Japan (they have to think about both since you can hit both in one turn once your fleet is parked in AK).
After the US wartime economy kicks in Japan can not trade ships and aircraft with the US and come out on top. The biggest problem I’ve seen with the US is getting that economy turned into combat effectiveness in the south pacific. Set this move up in the early stages of the game so Japan has to keep a watchful eye and counter measures tied up there. Then setup a decent fleet to head south which really puts Japan in big trouble.
All I’m saying is try this and see how much it cripples Japans ability to advance quickly.