NATO member states claim their troops will remain in Afghanistan regardless of whether President Biden honors the U.S.-Taliban agreement to withdraw all U.S. troops by May 1.

Pentagon spokesman John Kirby warned that unless the Taliban reduces violence, it is “hard to see a specific way forward” on fulfilling the deal’s stipulation for a full U.S. troop withdrawal. A new report from the Afghanistan Study Group, commissioned by Congress, also recommends postponing the U.S. withdrawal. These statements ignore a painful truth: NATO’s nearly 20-year effort in Afghanistan has proven that this war is unwinnable, at least at a cost the alliance is willing to pay. Continuing to fight is thus illogical, unwise, and immoral.

The U.S. military is the most powerful in the world. Yet, despite all of its drones, missiles, satellites, and special operators, it has proved unable to subdue a force of rural sneaker-clad insurgents in one of the poorest countries on earth. The two of us have a combined four combat deployments to Afghanistan since 2005. We have observed the Afghan war firsthand, at every level from theater headquarters down to infantry foot patrols. We have served when troop numbers and casualty levels were low and during the height of the 2010–2011 troop surge, when 150,000 U.S. and NATO troops were in Afghanistan. Through it all, no change in troop levels, tactics, or strategy has brought more than fleeting success. As much we wish to see the U.S. military succeed in Afghanistan, there are realities that render the U.S. effort futile.

The most fundamental challenge is that this war is primarily a civil war between several domestic Afghan factions and not a conventional fight between opposing armies. The U.S. military is very good at fighting identifiable uniformed opponents. It vanquished the Iraqi Army in 1991 and 2003, and it overthrew the Taliban regime at minimal cost. However, when the enemy is an insurgent, conventional U.S. military power is far less effective. America’s key military advantages are power projection, situational awareness via technology, and firepower. A grinding war among the people blunts all of these tools.

There are two basic routes to success in a counterinsurgency. The first is hyperviolent suppression, as Bashar Assad did in Syria and the Russians in Chechnya. As was said of the Romans, they “made a desert and called it peace.” The second is patient, methodical “population-centric counterinsurgency.” This is what NATO has claimed to be practicing in Afghanistan, drawing on (disputed) lessons from post-colonial campaigns in British Malaya, French Algeria, and elsewhere. The U.S., thankfully, has no stomach for the former course of action, and it lacks the patience, expertise, and will for the latter option. Put simply, victory over the Taliban is not in the cards.

The other critical factor is the battlefield situation in Afghanistan.

The Taliban have the wind at their backs. They control or contest more territory today than at any time since 2001. They continue to receive covert support from Pakistan. And both sides remain miles apart on even the items to be discussed, much less on finding solutions.

The Biden administration has a golden opportunity to conclude U.S. participation in the Afghan civil war. We are not delusional. We recognize that the Doha agreement with the Taliban is unlikely to end Afghanistan’s misery. But it does provide a “decent interval” for U.S. withdrawal. After 20 years of sunk costs and missed opportunities, that is the least bad outcome for the United States. America’s only vital interest in Afghanistan is preventing it from becoming a haven for international terrorism. The Taliban has pledged that they will not allow this. If they renege on this promise, the U.S. can punish and deter them, with means ranging from standoff missile strikes to a full-fledged punitive expedition.

Remaining engaged in combat operations as now will result in more needless casualties, cost billions of dollars, and will distract decision-makers from far more important threats to American security and prosperity. Let’s stick to the May 1 timetable.

Gil Barndollar is a senior fellow at Defense Priorities and served as an infantry officer in the United States Marine Corps, deploying to Afghanistan twice. Daniel L. Davis is a senior fellow for Defense Priorities and a former Lt. Col. in the U.S. Army who retired in 2015 after 21 years, including four combat deployments.

Article originally published at the Washington Examiner.