Pearl Harbor was of strategic importance in a potential war between the United States and Japan because that is where the American Pacific Fleet was stationed. President Franklin D. Roosevelt had commanded the fleet to move from California to Hawaii in 1940 because of heightened tensions between the two countries. The increase in tension was caused by Japan's imperial ambitions in the Pacific in general, and its continued aggression in China specifically. The United States and its allies placed a trade embargo on Japan in 1940 that was meant to harm the military ambitions of Japan. Vital materials like oil and steel were forbidden to be traded with Japan. After diplomatic efforts failed throughout 1941, Japan felt it made sense to strike at the United States first and commenced plans for an attack on Hawaii. The Japanese attacked Pearl Harbor on a Sunday morning because they felt that the troops would not be able to respond as well and the element of surprise would be the greatest.