World War II began after Germany's defeat in World War I. Adolf Hitler rose to power in Germany and built up the military, invading Austria, Czechoslovakia, Poland, and others. By 1940, Britain was the only Allied power left fighting Germany in Europe. Meanwhile, Japan had become increasingly militaristic and ambitious since the 1890s. In 1941, after the U.S. cut off oil exports, Japan attacked U.S. forces at Pearl Harbor and invaded territories across Southeast Asia like Malaya and Singapore. The British forces there were unprepared and underestimated the Japanese, resulting in a series of defeats for Britain.