April 1917. Germany's best weapon is a train ticket. Berlin puts Lenin in a sealed carriage like hazardous cargo and ships him across Germany toward Petrograd. The Tsar has already fallen. Lenin steps off at the Finland Station and tells the welcoming crowd that everything so far is not enough.
Why then
Three years of war have broken Russia's railways and starved its cities. In February bread riots and a garrison mutiny ended three centuries of Romanov rule; the Provisional Government that replaced it kept fighting. Lenin's promise of peace, land and bread lands on a country that wants all three.
What they wanted
Ludendorff needs a separate peace in the east. Lenin wants power for a party that will not share it. Kerensky, at the head of the Provisional Government, hopes for a democracy that can still fight. The Bolsheviks do not so much conquer Russia as pick it up where it was dropped.
What changed
In October the Bolsheviks seize the capital. A secret police, the Cheka, follows within weeks, and then a civil war. The century's longest experiment in remaking a country from the roots begins with a railway journey.
Still argued
Liberal historians see a coup, a party seizing power in a vacuum; others, a revolution from below that the Bolsheviks rode. German money mattered, and how much is still argued.