The US were angered by the Japanese invasion of Manchuria but did not do anything because she was following a policy of isolationism (not getting involved in other countries' affairs so that it could recover internally from the horrific casualties caused in the World War I) at that time. Sanctions of vital items like oil were placed on Japan. Attempts to settle things peacefully by holding talks between both countries were also made but proved to be all unsuccessful. However when Japan attacked the United States naval base in Hawaii, Pearl Harbour, on 7 December 1941, the very next day, the US declared war on Japan.

I would think that the main reason US entered the war due to national pride. She was the powerful country then and her position was currently being challenged. It would be a complete loss of face if US did not do anything in retaliation...