The Japanese attack on Pearl Harbor in the Hawaiian Islands on December 7, 1941 marked the official entrance of the United States into World War II.
During the 1930s, Japan, having already annexed Korea in 1910, sought to further expand its empire, particularly to gain natural resources. In 1931, the Japanese invaded Manchuria, a small, resource-rich province in northern China, and set up a puppet state called Manchukuo. In 1937, Japan invaded the rest of China and by some estimates killed as many as 300,000 people during the infamous Nanking Massacre. China would lose as many as 14 million people by the end of World War II.
Western powers were distressed by Japanese expansion, particularly because it violated the “Open Door” Policy supported by the League of Nations (a precursor to the United Nations), which had been put in place to ensure equal trading opportunities with China. The League of Nations reprimanded Japan, but this did nothing to stop its expansion.
