September 18, 1931: The Japanese engineer the Mukden Incident.
Similar to Germany’s Gleiwitz Incident, which was used to justify the 1939 invasion of Poland, the Mukden Incident served as the prelude to Japan’s invasion of Manchuria, which began the very next day.
On September 18, officers of the Imperial Japanese Army set off an explosion near a section of railway track owned by a Japanese company - the South Manchuria Railway; although damage was minimal (trains could reportedly still pass over this portion of track), the Japanese placed the blame for the attack on Chinese nationalists. On September 19, Japanese forces invaded and occupied Manchuria, establishing the puppet state of Manchukuo (满洲国) in September of 1932. The Japanese ignored the objections of the League of Nations, which would not have been able to enforce any of its mandates in the first place. The major western powers had little incentive and little authority to intervene militarily, and the state of the world economy was too poor to impose any significant economic sanctions. Therefore, Japan went mostly unopposed in its invasion, and the Manchukuo government remained in place until 1945, with Puyi, “the Last Emperor”, as its (puppet) sovereign.