To a certain extent, the conflict between the United States and Japan stemmed from their competing interests in Chinese markets and Asian natural resources. While the United States and Japan jockeyed peaceably for influence in eastern Asia for many years, the situation changed in 1931.
The answer on the American side is simple: the Japanese bombing of Pearl Harbor. The Americans were angry at the Japanese for their invasions of first Manchuria (1931), then China (1937), and later French Indochina (1940).
Therefore they were stunned when the unthinkable happened and Japanese planes bombed the U.S. fleet at Pearl Harbor on December 7, 1941. The following day, the United States declared war on Japan, and it soon entered into a military alliance with China.
Faced with severe shortages of oil and other natural resources and driven by the ambition to displace the United States as the dominant Pacific power, Japan decided to attack the United States and British forces in Asia and seize the resources of Southeast Asia.
Emperor Hirohito let it be known to General MacArthur that he was prepared to apologize formally to General MacArthur for Japan's actions during World War II—including an apology for the December 7, 1941, attack on Pearl Harbor.
What events contributed to a hostile relationship between Japan and the United States? War between Japan and the United States had been a possibility that each nation's military forces planned for in the 1920s, though real tension did not begin until the 1931 invasion of Manchuria by Japan.
Japan's surprise attack on Pearl Harbor would drive the United States out of isolation and into World War II, a conflict that would end with Japan's surrender after the devastating atomic bombing of Hiroshima and Nagasaki in August 1945. At first, however, the Pearl Harbor attack looked like a success for Japan.
Some officials were warned in clear detail by those with insider knowledge that Pearl Harbor was to be attacked by the Japanese — and some argue the attack could have been avoided altogether had these American officials heeded the warnings.