The latest crisis with Iran illustrates an important but widely neglected point about world politics: Amid all the talk about American decline, American power in the international system has actually grown. Even five years ago the U.S. could not force Iran out of world oil markets without causing a devastating spike in oil and gas prices that would destabilize the world economy. Today, world energy markets are so robust that Brent crude prices have fallen since the first set of attacks on oil tankers in May.

Simultaneously,...