In 1931, Japan invaded Manchuria, defeating the Republic of China's military forces and establishing Manchukuo, a puppet state. The Hoover administration deplored the invasion, but also sought to avoid antagonizing the Japanese, fearing that taking too strong a stand would weaken the moderate forces in the Japanese government and alienate a potential ally against the Soviet Union, which he saw as a much greater threat. In response to the Japanese invasion, Hoover and Secretary of State Stimson outlined the Stimson Doctrine, which held that the United States would not recognize territories gained by force.