But this presumes that the peace agreement with the Taliban will survive our departure if it comes in 2021 instead of in 2020. It won’t. Whether American troops leave in the next few months, next year, or later this decade, the Taliban are bound to make a strong push to take over Afghanistan once we’re gone — and they are very likely to succeed. The question then becomes whether this is an outcome we are willing to accept. If it is, it probably makes more sense to leave sooner rather than later — but only if Trump is willing and able to prepare the American people to accept this eventuality.

That is the challenge that confronts us. We either need to be willing to see Afghanistan fall to the Taliban — or be willing to keep troops in the country indefinitely. The latter option is the tripwire tactic that the U.S. first deployed after World War II, when we extended security guarantees to Western Europe, letting the Soviet Union know that any attempt to push the Iron Curtain further westward would trigger an American military response. We did the same thing a few years later with the armistice agreement between North and South Korea. Sixty-seven years later, the American tripwire (backed up by American troops) is still there in the Demilitarized Zone separating the two countries.