The reason the US sent an ultimatum in autumn 1941 which lead to the Japanese decision to attack Pearl Harbor was the Japanese actions in China. If the Japanese had given up on their China politics the US would have propably loosened their politics but cannot be sure. It seems FDR wanted to show Japan its place because his politics ( in my opinion ) did not leave Japan much breathing place except to stop their actions in China or...

Anyway, in my opinion, starting hostile actions anywhere even if not against the US troops would have started a war between the US and Japan quite soon because FDR had decided to stop Japan from growing into a bigger war machine.

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