China’s real problem isn’t the so-called Thucydides trap, which holds that a rising power like China must clash with an established power like the U.S., the way ancient Athens clashed with Sparta. It was Lenin, not Thucydides, who foresaw the challenge the People’s Republic is now facing: He called it imperialism and said it led to economic collapse and war.

Lenin defined imperialism as a capitalist country’s attempt to find markets and investment opportunities abroad when its domestic economy is awash with excess capital...