I tried the shuck-shuck approach while playing US and moved my Pacific fleet to the Atlantic (Japan didn't attack Pearl Harbor). However, Japan then used its entire fleet to invade Western United States. I quickly fell (due to a blunder), which decided the game.
How could the Allies have maximized on Japan's abandonment of the mainland? The UK lost its Indian Ocean fleet and didn't have resources to build a new one. Mainland Asia eventually came under British control, but there wasn't a chance to take Japan itself.
Some other information about the board state:
Germany destroyed the UK fleet on its first turn, then immediately followed the Japanese invasion of the US by destroying the entire American navy. Japan's navy sailed through the Panama Canal and captured London. Russia was still barely holding off Germany.