I admit that I havent studied this issue extremely closely (or seen it in use) but my sense of it is that as the Japanese player, I’m not all that worried about it. I mean you are going to have to fight the US forces SOMEWHERE. And if the US has 5 TRs + 5 tanks + 5 inf and enough navy to not make this be a one-time suicide force up North then that mean that the US has nothing harassing me in the SoPac/DEI areas. And honestly I’d rather take them on in my own front yard than all the way down away from my production and massed airpower.
Yes, the image of US tanks blitzing through China and liberating Chinese lands to spring up troops is somewhat disturbing, but I still think I would prefer that to having the US continually taking away my bonuses down south and occupying high-value islands. Also, having the US up north is keeping them out of support range from the Aussies and any surviving UK air/navy. That will deprive them of mutual support and threat potential.
And as was said above, it’s not like this is a surprise maneuver. The US has to built the Naval Base (giving 1 turn of warning) and then the TRs with all the associated troops etc have to be on hand to be there as well. If the US is massing that kind of invasion power in San Fran, believe me, I’m going to take counter-measures along the home front. :) The US may cause a bit of a mess up north, but I dont see it as being any ‘insta-win’ or anything close to it at this point.
All that said, I’ll give it a closer look in my next game as the US (and again as Japan) and see if this is something I think would be a worthy diversion of forces from what I consider the primary battle area of the DEI.