Socialism gained only after World War I disrupted incumbent powers and wealth. World Wars I and II really need to be considered as a unit. War continued into the '20s in Eastern Europe and then resumes in 1936 in Spain. See "War of the World" by Niall Ferguson.

Socialism was not that strong before WW I destroyed the power structures of the German, Russian, Austro-Hungarian, and Ottoman Empires. The resumption of war in World War II destroyed the Japanese and Chinese Empires and enervated the British and French sufficiently that the British and French Empires dissolved after the war.