Japan may attack French Indo-China at any time. It doesn’t require being at war with any Allied power, unless units belonging to an Allied power are located there.
@joephillips There is no provision in the rules of the game for “annexing” territories. If you want to create house rules for this, it’s up to you what happens; however, when it comes to a hostile power, “annex” is just a diplomatic word for “attack”.
I’m pretty sure BILL is right. Japan will be able to take the Dutch colonies without consequence. Otherwise there would be nothing neutral (except China) for Japan to attack and no need to keep the US out of the war for the first few turns.