It can be done both as a feint to tie up the US resources and for real if Japan wants to over-invest they can maybe pull it off, it is very risky though.

Wiping out the US is very hard but doable.

Correction- With Global it is only nine infantry.

True, but when I play America no one ever even thinks of trying to invade it. Later when it is time to start shipping the troops overseas they are all ready to go and you have flexibility to spend IPCs on the current map situation as the Joes are all ready to go. Since you are going to have to make them anyway you may as well make them early.

In the US normally I buy at least 10 infantry on turn one, sometimes more. I use these to replace those sent to reinforce the South and Alaska as well as the islands.

The next turn is spent buying at least six aircraft, to replace the ones that went OCONUS on turn one to backstop HI, AK  and Mexico.

If there is no attack on turn three then I buy a navy, at least three carriers and a couple of bullet stoppers.  I then send forces to reinforce Canada, activate Brazil (on the way to North Africa).

Russia is
1- 100% Infantry
2- 50% infantry/ 50% tanks
3- 25% infantry/ 75% tanks

Past turn three really depends upon what is going on.

We use a rack of poker chips. Much better than paper money and near indestructible.