There is a reason why it is very uncommon for Japan to invade the US- it is almost impossible to accomplish and can easily be exploited by a good US player. I would even go so far to say that the only advantage of invading the US is the element of surprise, but this alone is never enough to succeed against a good US player.
This Japanese strategy implies that Germany has to deal with ‘only’ UK and Russia.
The US should and will still be able to spend the vast majority of their IPCs (at least in the long run) in the Atlantic if Japan decides to invade, so I don’t agree with this statement. Germany will still have to contend heavily with the US.
Since there are a couple of ways the Japanese may conduct their attempt in invading the US you can’t give a precise US counter strategy- Are the Japanese building any capital ships? Subs? Are they just building transports (how many)? How strong did they attack Hawaii and what was the outcome? Are you playing with 1 or 2 hit BBs? Barring any outcomes far from normal, the US should still go with their typical purchase of 3 Atlantic transports and 4 EUS infantry. If the Japanese took Hawaii extra hard, then you may decide to purchase a combination of sub/s, inf and transport/s for the Pacific as early as turn 1.
I try to avoid the latter purchase strategy for 2 reasons. US units in the Atlantic are worth more than the equivalent in the Pacific. Also, as the US I want Japan to try to invade North America. Until Japan actually invades the US, the Japanese may still pull out of the mission and redirect their landings for mainland Asia or Australia/ New Zealand; the Japanese are not yet heavily invested in a battle front that they won’t win. As the US, I want Japan to commit those forces to North America as opposed to those other locations.
As a side note, as the US I suggest always completely pulling out of Alaska on turn 1 to increase US counter-attack potential and to remove Japanese BB bombardment effctiveness. I mention this because there was a discussion previously on ‘protecting Alaska.’ Alaska should not be protected (at least by leaving units there).
If anyone believes that there is a viable Japanese strategy to attacking the US, even if it’s just to remove the US from the European front, please let me know of any specifics including purchases and unit movement. I am very interested in seeing if there is actually a viable Japanese strategy against the US.