They wanted us in the war to fight the Germans would be my guess.
The Soviets didn’t warn the US, but no one knows why.
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One reason is that soon after Pearl Harbor, the Soviets transferred dozens of army divisions to the Eastern Front to help fight the Germans. The Japanese did the Russians a big favor when they attacked Pearl Harbor. They knew they were safe from Japanese attack so they could concentrate on the Germans.
The Germans sank the Reuben James on October 31, 1941, yet even that couldn't get us to declare war on Germany.