This month marks the anniversary of America's longest war: 17 years in Afghanistan. On this anniversary we must ask, why are we still engaged in what amounts to a forever war?

Even before he was a candidate for president, celebrity citizen Donald Trump tweeted in 2013 : "We should leave Afghanistan immediately" and "Let's get out of Afghanistan." But he's done a complete reversal as president.

In August 2017, President Trump acknowledged that his “original instinct was to pull out" but that "our nation must seek an honorable and enduring outcome worthy of the tremendous sacrifices that have been made, especially the sacrifices of lives." Indeed, the president declared, " We will fight to win,” but what President Trump needs to understand is that we can't win and, more importantly, we don't have to win.

According to FM3-24, the U.S. Army's counterinsurgency manual, "Twenty counterinsurgents per 1000 residents is often considered the minimum troop density required for effective COIN [counterinsurgency] operations." With a population of over 33 million, that means a force of 660,000 troops is needed in Afghanistan. For a sense of scale, consider that the total U.S. Army active duty force is less than 500,000 soldiers. It's also worth noting that that peak U.S. troop deployment during the Vietnam War was more than 500,000 soldiers and we did not win that counterinsurgent operation.

But even if the whole of Afghanistan doesn't need to be secured, it is still a bridge too far. According to the most recent Special Inspector General for Afghanistan Reconstruction (SIGAR) quarterly report, 11.6 million Afghans live in areas either under the control of, or contested by, insurgents. That equates to a requirement of 232,000 troops, which is more than double the peak U.S. troop deployment in Afghanistan of roughly 100,000 soldiers. And more than15 times more than current 15,000 troops.

But counterinsurgency is more than just troop levels. Successful counterinsurgency often requires the use of harsh—even brutal—and indiscriminate military force to impose security and order, such as the British used to put down the Mau Mau Rebellion in Kenya in the 1950s. The problem with such tactics is that they seem to increase resistance and fuel the insurgency—not to mention increasing anti-American sentiment throughout the Muslim world.

Even absent such tactics, military action intended to kill the enemy all too often results in killing innocent civilians, no matter how hard we try to avoid collateral damage. According to the United Nations Mission in Afghanistan (UNAMA), in the first six months of this year, there were 353 civilian casualties (149 deaths and 204 injured) due to air strikes—an increase of 52 percent for the same period in 2017.

The inevitable result of this collateral damage is the alienation of the civilian population, which creates more resentment of foreign forces and more sympathy for the insurgents. Indeed, according to FM 3-24, “An air strike can cause collateral damage that turns people against the host-nation government and provides insurgents with a major propaganda victory.”

If the U.S. were engaged in a war of national survival, we would be willing to accept the costs posed by engaging in counterinsurgent war. But Afghanistan is not a war of national survival. Neither the Taliban, nor al Qaeda, nor even ISIS in Afghanistan represents an existential threat to U.S. national security.

It's important to remember that the Authorization for the Use of Military Force (AUMF) approved by Congress on September 14, 2001, was “to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons.” In other words, Osama bin Laden, al Qaeda, and the then Taliban-led government in Afghanistan because they gave safe haven to bin Laden and al Qaeda.

The Taliban was driven from power in a matter of weeks. Over the next few years, al Qaeda’s senior leadership was disrupted and scattered—largely to neighboring Pakistan. And in May 2011, Osama bin Laden was found and killed by U.S. Special Operations Forces.

The war in Afghanistan is no longer about eliminating direct threats national security. Instead, it has become a costly exercise in democratic nation-building.

According to the Pentagon, the war in Afghanistan currently costs $45 billion per year. A report published last year by the Center for Strategic and International Studies states that Department of Defense (DoD) Overseas Contingency Operations (OCO) funding for the Afghan conflict from Fiscal Year (FY) 2001 to FY 2018 would be more than $840 billion. However, one estimate has the war costing more than $1 trillion to date and another estimates total war spending for Afghanistan at roughly $2 trillion.

As a candidate, Trump said, “If I become President, the era of nation-building will be ended.” President Trump would do well to remember his instincts and what he said before he became president. It's well past time to recognize that the threat in Afghanistan doesn’t warrant a continued U.S. military presence and the associated costs—which are not inconsequential.

Charles V. Peña is a senior fellow with Defense Priorities. He has more than 25 years of experience as a policy and program analyst and senior manager, supporting both the Departments of Defense and Homeland Security. Peña is the former director of defense-policy studies at the Cato Institute and author of Winning the Un-War: A New Strategy for the War on Terrorism.