What Trump’s Disgraceful Deal With the Taliban Has Wrought

Believing you’re uniquely capable of bending things to your will is practically a requirement for becoming president of the United States. But too often, in pursuit of such influence over foreign policy, presidents overemphasize the importance of personal diplomacy. Relationships among leaders can build trust — or destroy it — but presidents often overrate their ability to steer both allies and adversaries.

Ronald Reagan and Mikhail Gorbachev had built such a solid relationship that during the Reykjavik summit most of Reagan’s administration worried he would agree to an unverifiable elimination of nuclear...