Correct answer: Japan
History/context:
The open door policy meant all nations were to have equal access in trading with China - but Japan wanted to dominate in the region. In 1931, Japan invaded and occupied Manchuria, the northeastern region of China. The invasion followed an explosion that blew up a portion of railroad tracks near the city of Mukden. (Thus it became known as "The Mukden Incident.") The railway was owned by the Japanese, who had invested in development in the region. Japan blamed Chinese nationalists for the explosion, but others thought the bombing may have been done by Japanese military personnel to provide Japan with an excuse for invading and occupying Manchurian territory. The Japanese declared the region to be a new country, independent of China. which the Japanese called Manchuko. In reality, the territory was not independent but was controlled by the occupying Japanese army.
The League of Nations condemned Japan in 1933 for the events in Manchuria, but that did little to stop Japan. Japan withdrew from the League of Nations at that time, and by 1937 began further invasions into Chinese territory. and in the early 1940s Japan occupied French Indochina (territory in Southeast Asia).