Russia's allies, Britain and France, had long regarded the Tsars' autocracy as a medieval anachronism, and they welcomed the Provisional Government as a step toward constitutional rule. The government, first under Prince Lvov, and then under the Socialist leader Alexander Kerensky, promised to continue the war against Germany, and called for elections to a Constituent Assembly, which would meet the following year and become Russia's legitimate government. But Russia's new leaders soon found themselves opposed by the Petrograd Soviet, a "workers' council," which was dominated by Marxists–primarily Mensheviks, although a few Bolsheviks were allowed on the Executive Committee, among them Stalin and Lev Kamenev, both of whom would later serve on Lenin's Politburo.

But Lenin, who had still been traveling in Switzerland during the outbreak of the war, now found himself trapped there: a war zone lay between him and Russia. He could only watch helplessly as the opportunities for revolution–both in Russia and in Europe as a whole–seemingly slipped away. His subordinates in St. Petersburg (Petrograd) sat leaderless, and Stalin and Kamenev seemed ready to pursue a policy of reconciliation with their fellow Marxists–even going so far as to suggest Bolshevik support for the war effort, or at least an absence of open opposition to a continued struggle with Germany. They may have anticipated that a reunited Marxist party could work within the new political order.

Lenin, writing furiously from Switzerland, objected utterly to everything taking place in his absence: his letters and telegrams denounced both the Provisional Government and the Mensheviks, and criticized Stalin and Kamenev's tentative support for the new political order. Then came the intervention of fate–or geopolitics. The German government, which supported all possibilities of domestic unrest in their enemy nation, had been keeping a careful eye on Lenin for some time, and now arranged to ship the Bolshevik leader and a number of other revolutionaries, including Krupskaya, Inessa Armand, and even the Menshevik leader Martov, across Germany in a "sealed train" to Denmark, where they took ship for Sweden and then Finland. On April 16, Lenin arrived in St. Petersburg (Petrograd), where he was greeted by a large throng of soldiers, sailors, and workers; thus provided with a large audience, he proceeded to denounce the Provisional Government as being "deceivers." "Long live the worldwide socialist revolution!" he cried. The following day he issued his "April theses," in which he made explicit his intentions–to push for an overthrow of the Provisional Government and immediate Marxist revolution.

Had the Provisional Government possessed more strength, they might have arrested Lenin immediately. As it was, they waited three months, until the "July Days"– an abortive revolt of soldiers and sailors in the garrisons around St. Petersburg (Petrograd). After the government put down the uprising, it turned on the Bolsheviks–though they denied all involvement in the revolt–and Kerensky ordered the leading figures of the party arrested.

Lenin went into hiding, but by that time he was already well into an effective political campaign, which had four major goals: immediate peace with Germany; the redistribution of land to the peasantry; the transfer of industrial areas to the control of workers; and the recognition of the soviets as the supreme political authority in Russia. This "peace, land, and bread" program attracted considerable popular support to the Bolshevik cause, and Lenin fortified his growing strength by allying himself with the brilliant Leon Trotsky.

In order to counter the rising popularity of their rival, the Provisional Government began a propaganda campaign to brand Lenin as a German agent–a charge with more than a little truth in it. But while the campaign enjoyed a certain effectiveness, the Provisional Government was having serious difficulties holding onto power: the war was going badly, and a breach between Kerensky and a right-wing military commander, General Lavr Kornilov, led to an army uprising. The government put down the rebellion in September, but only with great difficulty. Lenin continued to urge immediate action to topple the tottering, Kerensky-led government, and by October Trotsky, who had been elected Chairman of the now-Bolshevik-controlled Petrograd Soviet, supported him. On October 23, Lenin emerged from hiding to attend a meeting of the Bolshevik Central Committee, where a vote was held for an immediate uprising.

It was by no means a unanimous decision–Kamenev and Grigory Zinoviev, both of whom would serve on Lenin's Politburo, opposed drastic action. But Lenin, with Trotsky (and Stalin) backing him, carried the day. On the night of November 6-7, 1917, Bolsheviks seized the train stations and electric plants in St. Petersburg (Petrograd), a rebellious warship aimed its guns at the Winter Palace (the site of the Provisional Government), and Kerensky fled from Russia, eventually taking refuge in the United States. They declared as Russia's new ruling body a "Council of People's Commissars," led by Lenin. Suddenly, the government of the world's largest country was constituted by a small revolutionary clique.