The Japanese attack on Pearl Harbor was the result of a decade of Tokyo's degrading relationship with the U.S, and the threat that Washington would enter World War II on the side of the Allies. The day after the attack, the U.S declared war on Japan, and three days later on Germany. A massive mobilization effort followed and a wave of anti-Japanese suspicion led President Roosevelt to pass an executive order that resulted in the internment of thousands of Japanese Americans in camps.