Russian Revolution
World War I absolutely wrecked Russia by 1917. Millions of soldiers were killed or wounded, whilst back home people were literally starving because of food shortages. When Tsar Nicholas II decided to personally lead the army (spoiler alert: bad idea), it just made everything worse.
By February 1917, people in Petrograd (Russia's capital) had completely had enough. Workers went on strike, people demonstrated in the streets demanding food, and crucially, the army refused to shoot the protesters. This February Revolution forced the Tsar to give up his throne after centuries of royal rule.
The Provisional Government stepped in to fill the power vacuum, but they made some seriously unpopular decisions. They kept Russia fighting in WWI when everyone wanted peace, and they refused to give land to the peasants who desperately needed it.
Vladimir Lenin and his Bolsheviks saw their chance and took it. Lenin's brilliant slogan "Peace, Land and Bread" promised exactly what people wanted: an end to the war, land redistribution, and food. In October 1917, the Bolsheviks stormed the Winter Palace, arrested the Provisional Government, and seized control of Russia.
Key Point: The Bolsheviks succeeded because they promised what people actually wanted, unlike the Provisional Government who ignored popular demands.
The new Bolshevik government immediately pulled Russia out of WWI by signing the Treaty of Brest-Litovsk with Germany. However, they then had to fight a brutal Civil War against the White Army from 1918. The Bolsheviks eventually won and became the Communist Party of the Soviet Union, ruling Russia for the next 70+ years.