Japan invaded Manchuria because they stated that the Chinese soldiers in Manchuria had ruined the Manchurian railway, which belonged to and was controlled by the Japanese.The invasion of Manchuria took place on September 18, 1931.
The Japanese had several locations in South Manchuria secured within days. The League of Nations gathered in April of 1932 to discuss Japan and the region, and agreed that the Japanese needed to leave. It was reported that the Japanese had taken over and were "exercising de-facto control in South Manchuria." In February of 1933, the League of Nations called a special assembly and voted that the Japanese must leave the area. However, the Japanese simply left the meeting and did not leave Manchuria.
There was not any country within the League of Nations that was prepared to place soldiers in the Manchuria area, and the group was unable to make a move. Shortly thereafter, on March 27, 1933, the Japanese told the League of Nations that they would be leaving the group. The Japanese did not leave Manchuria and continued to control it into the start of World War II, when the Soviet Union declared war on Japan by putting more than 1 million of their soldiers into Manchuria.