The Japanese occupation of Manchuria was the first real test of the League of Nations, as this was exactly the kind of aggressive act the League was created to prevent. Unfortunately, the League failed the test.
Ever since the eighth planet, Neptune, was discovered in 1846, some astronomers searched for a hypothetical ninth planet. Clyde Tombaugh discovered the ninth planet in 1930. Or did he?
Japan had a rough time of it in the 1920s, between the global economic shocks and a devastating 1923 earthquake. Japanese military commanders saw control of Manchuria as key to Japan's economic independence, and they weren't about to let the politicians in Tokyo interfere.