The bombing of Pearl Harbor surprised even Germany. Although
Hitler had made an oral agreement with his Axis partner Japan
that Germany would join a war against the United States, he was
uncertain as to how the war would be engaged. Japan's attack on
Pearl Harbor answered that question. On December 8, Japanese
Ambassador Oshima went to German Foreign Minister von Ribbentrop
to nail the Germans down on a formal declaration of war against
America. Von Ribbentrop stalled for time; he knew that Germany
was under no obligation to do this under the terms of the
Tripartite Pact, which promised help if Japan was attacked, but
not if Japan was the aggressor. Von Ribbentrop feared that the
addition of another antagonist, the United States, would
overwhelm the German war effort. But Hitler thought otherwise.
He was convinced that the United States would soon beat him to
the punch and declare war on Germany. The U.S. Navy was already
attacking German U-boats, and Hitler despised Roosevelt for his
repeated verbal attacks