Attack on Pearl Harbor
Japan had taken a heated stance when the United States opposed the Racial Equality Proposal to amend the Treaty of Versailles.  Despite this, the two nations were still friendly and traded. Tensions seriously grew with Japan's invasion of Manchuria in 1931.  Over the next decade, Japan expanded into China. This led to the Second Sino-Japanese War in 1937.  Japan spent great effort trying to isolate China.  They also tried to gain independent resources to attain victory on the mainland.  The "Southern Operation" was designed to assist these efforts by conquering Southeast Asia.