During the Cold War, Moscow supported the Latin American left because it naively thought communism was the wave of the future. These days, a more cynical Kremlin supports the Latin left because it knows it will fail. The dramatic confrontation taking shape in Venezuela—where dictator Nicolás Maduro clings to power with Russia’s help as the West declares opposition leader Juan Guaidó the legitimate president—is the first major crisis of this new era of great-power competition.

Why has Russia propped up Mr. Maduro? Because of...