@yazoinkergrapft Yes, the United States is allowed to declare war. If it does, the sea zone immediately becomes hostile, as there are enemy surface warships in it. However, since the United States is not declaring war until the Collect Income phase, during the Combat Move, Conduct Combat, and Noncombat Move phases the sea zone is still friendly, so the United States can still occupy it freely. On Japan’s next turn, its units in the sea zone will be starting the turn in an enemy-occupied sea zone, so the normal rules for that situation apply (those units must either move away in combat movement or attack).
Y