On the Pearl Harbor Attack: Often considered an immediate cause of the U.S. entering WWII, the Attack on Pearl Harbor marked a major short-term victory for the Japanese Imperial Navy. Early in 1941, President Roosevelt ordered the U.S. Pacific Fleet to be moved from San Diego to Pearl Harbor in an effort to thwart Japanese expansion throughout the region. Part of Roosevelt's reasoning here stemmed from the idea that Pearl Harbor was a shallow water lagoon, which would make torpedo attacks from planes very difficult. After a plane-dropped torpedo hits the water, it needs to reach an appropriate depth to achieve maximum speed, and thus, create mass destruction by hitting a ship's bow or stern. And although many of the torpedo strikes that sunk American battleships came from Japanese planes, the Imperial Navy also stationed numerous submarines near the harbor's entrance to deter escape. Yet when the U.S. ended its oil exports to Japan in July 1941 (to protest Japanese aggression in China), relations between the two nations had become largely toxic (although each side held non-aggression talks until late November).