In the late Twenties, Germany saw its Communist and National Socialist parties grow in popularity, especially the latter. Adolf Hitler became the unlikely leader of the right wing in Germany, while the Communists dismissed the Nazis as a temporary obstacle.
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.
Chiang Kai-shek unified China, more or less, but survival of the Nationalist government was threatened by warlords, the Japanese, and the Communists. Of these threats, Chiang regarded the Communists as the greatest.
The economic bad news had been centered in the United States, but in 1931, bank failures began multiplying in Europe, leading to the Great Depression.
The combination of higher reparations payments and a slower economy led the German government to pursue harsh and unpopular austerity policies.
The crash of the stock market was a shock, but what did it mean? No one was sure, but over the next 18 months, the US economy went from bad to worse.
There were voices warning that the stock market was overpriced, but even in October 1929, there were louder voices proclaiming that all was well, right up to the moment the bottom fell out of Wall Street.