Russia was one of the major powers involved in World War I, but when it came to negotiating for peace, they stayed out.

Russia had exited the war early. In 1917, the Bolshevik Revolution overthrew Tsar Nicholas and the Russian monarchy, and replaced that system with a radical style of Marxism favored by Vladimir Lenin. It was the Red Dawn—no, not the dumb movie, but the birth of Soviet Russia. (Source)

After World War I, Western leaders feared that similar Marxist revolutions would take place throughout Europe. Some countries, like Hungary, did eventually switch to Communism (source). Wilson hoped to keep Russia in the war by including favorable treatment of the Russian revolution in his Fourteen Points, which was praised by Lenin (source), Despite this, in 1918, the Bolsheviks had already signed a peace agreement with Germany (source).

While Wilson was focused on negotiating with the French, British, and Germans, Lenin's party, the Bolsheviks, was trying to consolidate its control over the country. As a result, the new Soviet Union was left out of the Treaty of Versailles, which delayed recognition of the new Communist state by the Western world. (Source)

And that, folks, is how we eventually got Dr. Strangelove.