Once you start looking, you find the boomerang at work in many surprising places. Trump’s flirting with a ban on Muslim immigration encouraged federal judges to encroach on executive power over visa policy. Firing FBI Director James B. Comey entrenched the Russia investigation far more deeply. Withdrawing from the Paris climate treaty spurred states from California to Virginia to toughen their policies on global warming. Threatening the research budget may have strengthened the National Institutes of Health’s hand in Washington. And so on.

The boomerang effect is no panacea. Trump can still do grave damage at home and abroad in the next 3½ years. If he undermined Obamacare, millions of people would suffer before we got to single-payer. Nationalist governments ensconced in parts of Eastern Europe could still draw strength from Trump. The absence of U.S. leadership in the world leaves ample ground for others to cause trouble.