In 1931, Japan invaded and conquered the part of China that is commonly known as Manchuria. This is a region in the northeastern part of China (some people define it to include areas that are part of Russia as well). The Japanese set up a puppet state in Manchuria. The puppet state was called Manchukuo. It remained as a Japanese puppet state until the end of World War II.
In 1931, Japan had an internationally-recognized right to run and to protect a railroad through Manchuria. For this reason, Japan had troops in Manchuria even though it was part of China. In September of 1931, rogue Japanese troops set off a very small explosion near to Japan’s railroad. The explosion was blamed on China and used as a pretext for the invasion. This incident is known as the Mukden Incident.