The military history of the Soviet Union began in the days following the 1917 October Revolution that brought the Bolsheviks to power. In 1918 the new government formed the Red Army, which then defeated its various internal enemies in the Russian Civil War of 1917–22. The years 1918–21 saw defeats for the Red Army in the Polish–Soviet War (1919–21) and in independence wars for Estonia (1918–20), Latvia (1918–20) and Lithuania (1918–19). The Red Army invaded Finland ; fought the Battles of Khalkhin Gol of May–September 1939 against Japan and its client state Manchukuo; it was deployed when the Soviet Union, in agreement with Nazi Germany, took part in the invasion of Poland in September 1939, and occupied the Baltic States, Bessarabia and Northern Bukovina. In World War II the Red Army became a major military force in the defeat of Nazi Germany and conquered Manchuria. After the war, it occupied East Germany and many nations in central and eastern Europe, which became satellite states in the Soviet bloc.

Lev Davidovich Bronstein, better known as Leon Trotsky, was a Russian revolutionary, Soviet politician, journalist, and political theorist. He was a central figure in the 1905 Revolution, October Revolution, Russian Civil War, and the establishment of the Soviet Union. Alongside Vladimir Lenin, Trotsky was widely considered the most prominent Soviet figure and was de facto second-in-command during the early years of the Russian Soviet Republic. Ideologically a Marxist and a Leninist, his thought and writings inspired a school of Marxism known as Trotskyism.