United states of America was politically neutral for most of the time duration of the World War II. It was however supporting the Allied powers in a very covert manner.
This agitated the Germans as the supplies were keeping the allies in the war. The Germans continued to sink U.S. merchant ships taking supplies to England which enraged the Americans.
The tip of the iceberg was reached when the Japanese (one of the member of the Axis power) attacked a very important naval base in the Pearl Harbor on 7th December 1941. Three days later, the United States of America declared war against Germany, Italy and Japan.
To know more about Pearl Harbor here:
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