December 7, 1941 (or 6, 1941, depending on which time zone you were in), was the day of the Battle of Pearl Harbor, in which Japan broke political ties with the US, and attacked the US harbors in Pearl Harbor, destroying multiple infrastructure and battleships before the US had time to react. This battle also sparked the US to declare war on Japan, and led to the internment of Japanese American Nisei and Issei in an attempt to rid the US of spies & political enemies.
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In the American Revolutionary War, the British were fighting for themselves against the Americans and, further into the war, the French.
I think that Hitler saw in Austria the chance to increase the number of loyal, German soldiers - Austrians speak German and are culturally similar so it was easy to include them into the German army and so quickly acquire a great number of loyal soldiers who spoke the language and shared the culture with the Germans.
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Answer:
America was working on themselves and did not want to join someone else's war.
When Pearl Harbor was bombed that was when America joined the war
