“We know only too well that war comes not when the forces of freedom are strong but when they are weak,” declared Ronald Reagan at the 1980 Republican National Convention. “It is then that tyrants are tempted,” he added, summing up his conviction that peace could only be secured through strength. Until Donald Trump followed in Reagan’s footsteps as the GOP’s standard-bearer, this formulation was axiomatic on the American right. But Trump’s careless rhetoric — which often flirted with isolationism and flattered America’s sworn enemies in Moscow and Pyongyang — emboldened the Republican Party’s isolationist wing. In an op-ed endorsing...