Which event is associated with Japan in 1931?

The event associated with Japan in 1931 is the invasion of Manchuria. This marked a significant turning point in Japanese expansionism and militarism in the early 20th century. In September 1931, the Japanese military staged a false flag incident known as the Mukden Incident, which they used as a pretext to invade Manchuria. This aggressive action led to the establishment of the puppet state of Manchukuo, demonstrating Japan's imperial ambitions and disregard for international norms.

This invasion was a beginning of Japan's military expansion throughout Asia that would escalate into further conflicts leading up to World War II. The event illustrated the failures of the League of Nations to effectively respond to acts of aggression, as Japan faced minimal consequences for its actions, eventually leading to its withdrawal from the League in 1933.

The other choices reflect events that are either unrelated to Japan's actions in 1931 or occurred at different times. The Kellogg-Briand Pact, signed in 1928, aimed at preventing war, while the Tokyo War Crimes Tribunal was established after World War II to prosecute Japanese leaders. Thus, the invasion of Manchuria is the pivotal event that is distinctly tied to Japan in 1931.