Known officially as the Treaty of Non-Aggression Between Germany and the Union of Soviet Socialist Republics, the Hitler-Stalin pact—signed 80 years ago Friday—stunned the world. Some said that by uniting implacable ideological foes it had turned isms into wasms.

That wasn’t quite right. As German negotiator Karl Schnurre had observed the month before, “despite all the differences in their respective worldviews, there is one common element in the ideologies of Germany, Italy and the Soviet Union: opposition to the capitalist...