In September 1931 Japan controlled the Manchurian railway and claimed that China had sabotaged the railway.
By February 1932, Japan had conquered Manchuria
China went to the League of Nations for help after Japan invaded Shanghai.
A commission was led by Lord Lytton in 1932 to see what was happening and declared that Japan should leave China. However the League couldn't agree on punishments
Britain and France weren't ready to send an army. Japan stayed in Manchuria and later on in July 1937, Japan also invaded China.