One sneak attack before US enters war. On any enemy you choose, in any territory. It is only the first round of combat.
Japanese Sneak Attack: Japan is allowed to use a sneak attack once, before the United States declares war, on any country that Japan can declare war against. In the first round of combat, all defending units roll at a “1”. This Sneak Attack must be used on the round that Japan declares war against the defending player, and must be declared after all combat moves are announced.