Obama then redoubled this unforced error by announcing an even more draconian drawdown of U.S. troops in 2014, not on the basis of on-the-ground military conditions, but with the political timetable of getting all troops out by the end of his own tenure. The predictable result was that security conditions deteriorated, eventually forcing the White House to stop short of the complete withdrawal it had promised, but only after U.S. forces had been severely pared back, the Taliban had reclaimed momentum, and regional powers had stepped up their support for insurgents in anticipation of a post-American Afghanistan.

What Washington has never attempted in Afghanistan, over the course of more than 15 years there, is the one policy that has been necessary from the outset: an explicit commitment to a sustainable, sustained U.S. military presence in the country.

Making such a commitment would send the unequivocal message to the Taliban that it cannot hope to prevail on the battlefield and must therefore pursue political reconciliation seriously. It would also position America for the tough diplomacy to convince Afghanistan's neighbors, foremost Pakistan, to stop backing insurgent groups in preparation for an American exit.

The strategic paradox of Afghanistan is that the more the United States has sought to leave, the more it has fostered the conditions that have forced it to stay. By contrast, the sooner Washington can convince all parties to the conflict of its long-term intent to remain, the sooner it can set the conditions to drive the conflict towards an end game.

To be clear, a sustained U.S. military presence in Afghanistan alone is no guarantee of success. But repeating the mistakes of the past by trying to withdraw troops from the country is a surefire recipe for more failure.

Can Americans stomach an open-ended military commitment to Afghanistan? Didn't they, after all, elect Trump—and for that matter, Obama—in part because they promised to diminish America's overseas burdens? Won't they demand a date by which all of U.S. forces come home?

This is, in some respects, a strange argument. More than 60 years after the end of the Korean War, tens of thousands of American troops are still deployed there—in the shadow of Kim Jong Un's arsenal—without any hint of domestic controversy, because Americans long ago accepted that this was in the national interest. So too with the enduring U.S. military presence in Europe and Japan after World War II, and across the Middle East since the early 1990s.

In truth, the foremost responsibility of any president is to keep Americans safe. Preventing Afghanistan from once again becoming a terrorist sanctuary from which attacks on America can be launched is as clear-cut a vital national interest as any in the world. If the price for this is a sustained military presence there—and the alternative, withdrawal, is more likely to result in a terrorist victory along the lines of what happened in Iraq after America left—that is not seemingly a prohibitively difficult case to make to the American people. On the contrary, it is telling that, almost 16 years after 9/11, there is no great groundswell of public protest or opposition to America's current operations in Afghanistan. In a perfect world, of course, U.S. forces wouldn't be required to stay in Afghanistan—or anywhere else for that matter—but as Americans long ago internalized, that is not the world they live in.