OK thanks for the help!
The rulebook says that the US can declare war on japan in a turn that japan has attacked a Allied territory (not including china.) so, can they declare war on japan right in US1??
If Japan declares war on an Allied power other than China in its first turn, the US can go to war on its first turn.
See the FAQ for important corrections to the poltical rules.
yeah, its my mistake. the rulebook says that the US gets their 40 ipc boost in the turn after a japanse attack on a allied territory. China is not included