Prelude to the attack on Pearl Harbor

Before the attack on Pearl Harbor, war between the Empire of Japan and the United States was a possibility each nation's military forces had planned for after World War I. The expansion of American territories in the Pacific had been a threat to Japan since the 1890s, but real tensions did not begin until the Japanese invasion of Manchuria in 1931. Japan's fear of being colonized and the government's expansionist policies led to its own imperialism in Asia and the Pacific, as it sought to join the great powers, all of which were Western nations.

Source: Wikipedia — Prelude to the attack on Pearl Harbor (CC BY-SA 4.0)