Bolshevik fighters, first detachment of the Red Army and former workers of "Triangle" factory, pose with their guns in November 1917 in Petrograd. March 1917 Russian Revolution started with demonstrations against the war and the food supply shortages in Petrograd and led to the abdication of Tsar Nicholas II. During the October 1917 Revolution, the Bolsheviks overthrew the Kerensky's Provisional Government, formed during the March 1917 Revolution, and replaced it for a soviet one, leading to the establishment of the Soviet Union. The October Revolution was led by Vladimir Lenin and Leon Trotsky, and marked the beginning of the spread of communism in the twentieth century.