The decades of the 1930s and 1940s were the most tumultuous ever in the history of the uneasy relationship between Japan and the United States. Japan was a militaristic society determined to establish its dominance in Southeast Asia. America was an isolationist country desperate to keep out of war. But war came. Through it, America and Japan were transformed from enemies to steadfast allies.

Ominous Events

In the 1930s Japan was a heavily populated island short of natural resources and dependent on trade for prosperity. It sought to enhance its access to resources by influence in China. It invaded Manchuria in 1931. The conflict erupted into full-scale war in 1937. Still recovering from World War I, the United States and other western powers sought to avoid another war. The United States responded to Japan by enacting weak economic sanctions and offering some economic and military aid to China.

Rising Tensions

Rather than persuading Japan to scale back its military ambition, American sanctions inflamed it all the more. The United States responded by tightening sanctions. Japan was dependent on oil from America. When the United States cut off the supply, Japan began seriously contemplating war. Impressed by its German ally's success with "blitzkrieg" attacks, it began to consider plans for its own sudden, overwhelming attack on America.

Pearl Harbor

On December 7, 1941, Japan struck with a devastating air attack on the American fleet anchored at Pearl Harbor, Hawaii. The United States and Japan were technically at peace at the time. Until 1999, most historians thought the Japanese Embassy in Washington bungled the notification of the declaration of war before the attack. A cache of newly found diplomatic papers that year suggested the sneak attack was intentional. Either way, it led to a declaration of war by the United States within days. Because Japan had a military alliance with Nazi Germany, America's declaration of war against Japan automatically brought America into the war in Europe, as well.

Atomic Bomb

Despite the defeat of its German and Italian allies in Europe in early 1945, Japan vowed to fight on. The Potsdam Declaration, which ended the war in Europe, threatened Japan with utter destruction if it did not surrender. In early August 1945, American bombers dropped leaflets over Hiroshima warning of a cataclysmic new type of bomb that would be used shortly on the city. On the morning of August 6, the first atomic bomb was dropped on Hiroshima. The destruction was not enough to force surrender, so a few days later another was dropped on Nagasaki. On August 15, 1945, Japanese Emperor Hirohito announced Japan's surrender.

Occupation of Japan

America and its allies decided well before war's end that Japan would have to be radically restructured to avoid future wars. Its governmental system was to be transformed into a democracy, with all former generals barred from holding office. The economy was restructured by ending the semi-feudalism that prevailed and introducing trade unions. Big farms were broken up and the land redistributed to small farmers. General Douglas MacArthur headed the occupation, which lasted for seven years. The occupation policies proved so successful that Japan became the United States' most steadfast ally in Asia and one of its strongest trade partners.