Starting in 1931, tensions escalated. Japanese actions against China in 1931 and especially after 1937 during the Second Sino-Japanese War caused the United States to cut off the oil and steel Japan required for their military conquests. Japan responded with attacks on the Allies, including the surprise attack on Pearl Harbor on December 7, 1941, which heavily damaged the US naval base at Pearl Harbor, opening the Pacific theater of World War II. The United States made a massive investment in naval power and systematically destroyed Japan's offensive capabilities while island hopping across the Pacific. To force a surrender, the Americans systematically bombed Japanese cities, culminating in the atomic bombings of Hiroshima and Nagasaki in August 1945. Japan surrendered, and was subjected to seven years of military occupation by the United States, during which the American occupiers under General Douglas MacArthur eliminated the military factor and rebuilt the economic and political systems so as to transform Japan into a democracy.