In September 1931, the Japanese claimed that Chinese soldiers had sabotaged the Manchurian railway in Korea, which Japan controlled. Japan attacked and by February 1932, had brutally conquered Manchuria.
Meanwhile, in January-March 1932, Japan attacked and captured the city of Shanghai in China itself. In March 1932, China appealed to the League of Nations.
In April 1932, a League delegation led by Lord Lytton arrived in Manchuria to see what was happening, and in October 1932 it declared that Japan should leave. In February 1933, a special assembly of the League voted against Japan, so the Japanese walked out.
The League, however, could not agree on sanctions, and Britain and France were not prepared to send an army. Not only did the Japanese stay in Manchuria, but in July 1937 they also invaded China.