The Bolshevik Revolution
People had tried revolutions in Russia before 1917 but they had failed. The Czars ruled the country and the serfs were treated incredibly poorly. But with Europe in the throes of World War One, the Bolsheviks rose up against the aristocracy and turned the world’s biggest country into the first communist state.
The Bolshevik revolutions is important for a few reasons. Not only was it the first realization of Communism as applied to a large nation state, but it toppled one of the oldest dynasties in the world and helped to change the course of the First World War. Not only were Russia no longer fighting in the war, but the country briefly descended into civil war and other countries feared the spread of communism so much that they sent troops to fight the Bolsheviks.
Without the Bolshevik Revolution, we would not have had the USSR, the Cold War, the cosmonauts forays into space, and a thousand other major events in recent world history.