Beginning in 1868, Japan undertook political, economic, and cultural transformations emerging as a unified and centralized state, the Empire of Japan. This period was a time of rapid economic growth. Japan became an imperial power, colonizing Korea and Taiwan. Starting in 1931 it began the takeover of Manchuria and China, in defiance of the United States. Escalating tension with the U.S.--and western control of Japan's vital oil supplies—led to World War II. Japan launched multiple successful attacks on the U.S. as well as British territories in 1941–42. After a series of great naval battles, the Japanese sank the U.S. fleet and largely destroyed its largest west coast cities through air raids.