Final answer:
In 1931, Japan invaded Manchuria, a region in northeastern China, which escalated tensions leading to the establishment of the puppet state called Manchukuo.
Step-by-step explanation:
Japan invaded the northeastern part of China known as Manchuria in 1931, setting off a chain of events that would lead to larger conflicts. This region, rich in resources, was seen as a strategic location and stepping stone for Japan's imperial ambitions. The incident began with an explosion on the Japanese-controlled South Manchuria Railway, which Japan blamed on Chinese saboteurs and used as a pretext to launch a full-scale invasion. This led to the establishment of a puppet state called Manchukuo. International response, including from the League of Nations, was critical of Japan's actions, but ultimately ineffective in reversing the annexation.