Trotsky was initially a supporter of the Menshevik Internationalists faction of the Russian Social Democratic Labour Party. He joined the Bolsheviks immediately prior to the 1917 October Revolution, and eventually became a leader within the Party. During the early days of the Soviet Union, he served first as People's Commissar for Foreign Affairs and later as the founder and commander of the Red Army as People's Commissar of Military and Naval Affairs. He was a major figure in the Bolshevik victory in the Russian Civil War (1918–20). He was also among the first members of the Politburo.
After leading a failed struggle of the Left Opposition against the policies and rise of Joseph Stalin in the 1920s and the increasing role of bureaucracy in the Soviet Union, Trotsky was successively removed from power in 1927, expelled from the Communist Party, and finally deported from the Soviet Union in 1929. As the head of the Fourth International, Trotsky continued in exile in Mexico to oppose the Stalinist bureaucracy in the Soviet Union. An early advocate of Red Army intervention against European fascism, in the late 1930s, Trotsky opposed Stalin's non-aggression pact with Adolf Hitler.
Stalin assigned the organisation and execution of a plan to assassinate Trotsky to Nahum Eitingon who recruited Ramón Mercader during the Spanish Civil War.
On 24 May 1940, Trotsky survived a raid on his home by armed Stalinist assassins led by GPU agent Iosif Grigulevich, Mexican painter and Stalinist David Alfaro Siqueiros, and Vittorio Vidale. Trotsky's young grandson, Vsievolod Platonovich "Esteban" Volkov (born 1926), was shot in the foot and a young assistant and bodyguard of Trotsky, Robert Sheldon Harte, was abducted and later murdered, but other guards saw off the attack.
On 20 August 1940, Trotsky was attacked in his home in Mexico with an ice axe by undercover NKVD agent Ramón Mercader. The blow to Trotsky's head was poorly delivered and failed to kill Trotsky instantly, as Mercader had intended. Witnesses stated that Trotsky spat on Mercader and began struggling fiercely with him. Hearing the commotion, Trotsky's bodyguards burst into the room and nearly killed Mercader, but Trotsky stopped them, laboriously stating that the assassin should be made to answer questions.
Trotsky was taken to a hospital, operated on, and survived for more than a day, dying at the age of 60 on 21 August 1940 as a result of blood loss and shock. Mercader later testified at his trial:
I laid my raincoat on the table in such a way as to be able to remove the ice axe which was in the pocket. I decided not to miss the wonderful opportunity that presented itself. The moment Trotsky began reading the article, he gave me my chance; I took out the ice axe from the raincoat, gripped it in my hand and, with my eyes closed, dealt him a terrible blow on the head.
According to James P. Cannon, the secretary of the Socialist Workers Party (USA), Trotsky's last words were "I will not survive this attack. Stalin has finally accomplished the task he attempted unsuccessfully before."