At the end of the First World War, the Allies intervened in the Russian Civil War, originally from the American perspective to assist the Czechoslovak Legion, though after the victory against Germany the allies clearly sided with the "Whites" against the Bolsheviks. To be fair, this was not yet the Soviet Union, and would not be until after the Allies withdrew and the Bolsheviks won the Russian Civil War, but it was still not a good start to Western relations with the USSR.

After the Bolsheviks won the civil war, Vladimir Lenin implemented the New Economic Policy (NEP), which allowed for some measure of private ownership and profit. Relations with the West normalized to a degree, and the Great Powers of Europe extended diplomatic recognition to the Soviets in the 1920s and early 1930s (the United States did not recognized Bolshevik rule until after the rise of Hitler). Germany and the USSR began to work closely together during the 1920s, as The Weimar Republic helped to modernize Soviet industry and military. And although the government of the United States continued to officially shun Moscow, American businessmen took advantage off the NEP and the American government supported their activity in Russia (but not always--this is a complicated topic, see Katherine A.S. Siegel. Loans and Legitimacy: The Evolution of Soviet-American Relations, 1919-1933).

Things turned for the worse after Stalin came to power. He overturned the NEP, began to forcibly collectivize the peasants, and infamously purged his government through a series of executions. Western fears reignited, and the Soviet Union was again perceived as an outcast. This was very important during the Sudeten Crisis and the subsequent Munich Conference, as the Soviet Union and Stalin were not consulted or invited to the conference. The West just could not accept the violent, unstable Marxist as any ally. Stalin, for his part, pushed for harsher measures against Hitler. When the West refused to work with him, he famously struck a deal with Hitler to divide up Poland. It is safe to say that Western perceptions of the Soviet Union, whether reasonable or unreasonable given Stalin's megalomania and the Great Purges, contributed to early Hitler triumphs in Czechoslovakia and Poland, as Stalin gave up on forming an alliance with the West and moved instead to capitalize on Hitler's aggression.