Your question is a little unclear. However, I believe i have the answer you are looking for.
If Japan attacks the US, the UK or ANZAC without the UK or ANZAC player attacking first, the US goes to war with Japan. If Japan occupies any Dutch territory the US goes to war with Japan, except for one situation. If the UK or ANZAC occupy a Dutch territory, and then attack Japan, Japan is allowed to take that Dutch territory from the Allies without bringing the US into the war. If the Japanese do not attack the US before the beginning of the third US turn, the US go to war with Japan at the beginning of their third collect income phase, and begin collecting the additional 40 IPCs.
I hope this helps.
I recommend you read the FAQ thread at the top of this section also. Good luck.