The idea that the U.S. can leave if the Taliban promise to combat rather than conspire with these groups is also wrongheaded. Until the Taliban demonstrate they have both the determination and the capability to work with the Afghan government against international terrorists—and there is ample reason to doubt this—common sense dictates the U.S. must retain its own means to pressure extremist networks plotting against the American homeland and U.S. allies. This can be accomplished only by having some number of capable American forces in Afghanistan, along with substantial “enablers” such as unmanned aerial vehicles and close air support.

While Iraq’s sectarian unraveling after the U.S. withdrawal in 2011 was a possibility that some foresaw, it was far from assured. If the Trump administration orders a full pullout from Afghanistan, there is considerably less doubt about what will happen—full-blown civil war and the re-establishment of a terrorist sanctuary as existed when the 9/11 attacks were planned there.

The Taliban have clearly indicated what they will try to do once U.S. forces are gone: overthrow the Afghan government and reimpose medieval rule. Their resistance to a formal cease-fire, continued barbaric attacks on civilians, and opposition to elections scheduled for this fall are all warning signs. Such a conflagration is likely to reinvigorate the flagging fortunes of Islamist extremism world-wide and the global terrorist threat—which, despite the destruction of Islamic State’s territorial caliphate in Iraq and Syria, is by no means defeated.