October 1917: Russian workers take power.
November 1917 to summer 1921: The Russian workers' state fights for its life in civil war against counter-revolutionaries, peasant revolts, and 14 foreign armies.
1923 to 1927: Trotsky leads the Left Opposition against the rising Stalinist bureaucracy. Trotskyists and dissidents purged from many Communist Parties outside Russia.
December 1927: Defeat of the Left Opposition in Russia. Trotsky's allies Zinoviev and Kamenev capitulate immediately; Trotskyists sent to exile in remote parts of the USSR.
January 1928 to early 1930: Stalin launches (waveringly at first) a new economic course, to forced collectivisation and forced-march industrialisation; and crushes all life in the trade unions and the Bolshevik party.
January 1929: Trotsky deported from USSR. Until his death in 1940, Trotsky will be evicted from one country after another.
April 1930: First international conference of the Trotskyist movement (seen at first as an international grouping of expelled factions of Communist Parties).
1933: After the German Communist Party's collapse in the face of Hitler's seizure of power, and the failure of the Communist Parties to react, the Trotskyists turn to building a new International and advocating a new workers' ("political") revolution in the USSR.
1934-8: Great Terror in the USSR. All known Trotskyists, and most surviving Bolsheviks, are wiped out by Stalinist repression.
August 1939: Hitler and Stalin sign military pact, followed by almost simultaneous invasions of Poland by Hitler (seizing the west) and Stalin (seizing the east). Debate among Trotskyists about how to react.
November 1939: Stalin invades Finland. Meets fierce resistance. The debate among the Trotskyists is intensified by dispute how to react to this new invasion.
April 1940: The US Trotskyist movement, which as the Nazis sweep across Europe soon will become almost the only sizeable Trotskyist movement in the world able to operate openly, splits after the dispute on Finland and Poland. James P Cannon leads one faction, Max Shachtman another. Trotsky backs Cannon.
August 1940: Stalinist agent murders Trotsky.
1941: The majority of Shachtman's grouping, the Workers' Party, moves to seeing the USSR as a "bureaucratic collectivist" state rather than a "degenerated workers' state".
1943-5: With USSR's victory at Stalingrad and advance into eastern and central Europe, differences between the "orthodox" Trotskyists (Cannon) and the heterodox (Shachtman) sharpen.
1946-7: Temporary rapprochement between the two Trotskyist currents, as the Cannon group takes a sharper anti-Stalinist line under pressure from Trotsky's widow Natalia Sedova. But reunification talks fail.
From 1948: after the outbreak of open conflict between Yugoslav Stalinism (Tito) and Stalin, the "orthodox" Trotskyists start hailing Stalinist states outside USSR as "deformed workers' states", deficient in democracy but still expressions of an advancing "world revolution".
Late 1953: The "orthodox" Trotskyists split, as a section of them, led by Cannon, recoil and strive for a sharper anti-Stalinist line.