Although Japan had begun to take a hostile policy against the United States after the rejection of the Racial Equality Proposal, the relationship between the two countries was cordial enough that they remained trading partners. Tensions did not seriously grow until Japan's invasion of Manchuria in 1931. Over the next decade, Japan expanded into China, leading to the Second Sino-Japanese War in 1937. Japan spent considerable effort trying to isolate China and endeavored to secure enough independent resources to attain victory on the mainland. The "Southern Operation" was designed to assist these efforts.