The policy of expansionism in China pursued by the autonomous Kwangtung Army of Japan accelerated in the late 1920s and early 1930s and became a major concern of the U.S. Government. On September 18, 1931, Japanese soldiers guarding the South Manchurian Railway blew up part of the track in order to manufacture an excuse to seize Manchuria proper. Secretary of State Henry L. Stimson reacted to what he regarded as a violation of international law as well as treaties that the Japanese Government had signed. Since calls for a cessation of hostilities between China and Japan failed and President Herbert Hoover had rejected economic sanctions in principle, Stimson declared in January 1932 that the U.S. Government would not recognize any territorial or administrative changes the Japanese might impose upon China. The Stimson Doctrine was echoed in March 1932 by the Assembly of the League of Nations, which unanimously adopted an anti-Japanese resolution incorporating virtually verbatim the Stimson Doctrine of nonrecognition. However, as the Secretary of State later realized, he had at his disposal only "spears of straws and swords of ice." In short order, Japanese representatives simply walked out of the League, and the Kwangtung Army formalized its conquest of Manchuria by establishing the puppet state of Manchukuo under former Chinese emperor Pu-Yi. When war between Japan and China broke out following a minor clash between military units at the Marco Polo Bridge in 1937, the impotence of the "Stimson Doctrine" became even more apparent.