Trotskyism is the theory of Marxism as advocated by Leon Trotsky. Trotsky identified himself as an orthodox Marxist and Bolshevik-Leninist, and supported founding a vanguard party of the proletariat, proletarian internationalism, and a dictatorship of the proletariat based on working class self-emancipation and mass democracy. Trotskyists are critical of Stalinism, as they oppose Stalin's theory of Socialism in One Country in favor of Trotsky's theory of Permanent Revolution. Trotskyists also criticize the bureaucracy that developed in the Soviet Union under Stalin.
Vladimir Lenin and Trotsky were close both ideologically and personally during the Russian Revolution and its aftermath, and some call Trotsky its "co-leader". Trotsky was the paramount leader of the Soviet Red Army in the direct aftermath of the Revolutionary period.
Trotsky originally opposed some aspects of Leninism. Later, he concluded that unity between the Mensheviks and Bolsheviks was impossible, and joined the Bolsheviks. Trotsky played a leading role with Lenin in the revolution. Assessing Trotsky, Lenin wrote, "Trotsky long ago said that unification is impossible. Trotsky understood this and from that time on there has been no better Bolshevik."