Almost immediately after President Trump directed the Pentagon withdraw all U.S. troops from Syria, he ordered a reduction of U.S. forces in Afghanistan—more than 7,000 soldiers, which is about half of the current force. Given the hysterical reaction of Washington’s foreign policy establishment about the president’s Syria decision, there will certainly be an even more shrill “the sky is falling” chorus regarding Afghanistan. But whatever one thinks of the president’s temperament, his management style, and the decision-making process (or lack thereof), his decision about Syria was the right thing to do, and so is his decision to pull back in Afghanistan. After more than 17 years, it’s well past time to wind down America’s longest war.

It is important to remember the original mission in Afghanistan authorized by Congress 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, Usama 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, Usama bin Laden was found and killed by U.S. Special Operations Forces. So the original mission has been long since accomplished.

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Yet along the way, well before bin Laden was killed, the mission morphed into propping up the fledgling Karzai government in Kabul. As such, the U.S. military effort was no longer about going after those who were responsible for 9/11, but became an exercise in democratic nation-building.

But—however desirable a representative, multiethnic, democratic government in Afghanistan would be—it is not an absolute necessity for U.S. national security. The overriding requirement is that whatever government controls Afghanistan, it understands that the United States will not tolerate support for or the harboring of any terrorist group with global reach that directly threatens the United States—even if such a government is not a friendly government. Al Qaeda and ISIS are terrorist threats in Afghanistan, but neither are a direct, existential threat to the American homeland.

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Moreover, we can’t commit the military resources required to fight a war in Afghanistan, a fact that policymakers have been unwilling to acknowledge or discuss with the American public.

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, a force of 660,000 troops would be needed in Afghanistan. For a sense of scale, the total U.S. Army active duty force is less than 500,000 soldiers. It's also worth noting that peak U.S. troop deployment during the Vietnam War was more than 500,000 soldiers and we did not win that counterinsurgent operation.

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 would require 232,000 troops, which is more than double the peak U.S. troop deployment in Afghanistan of roughly 100,000 soldiers and some 15 times more than the current force in Afghanistan.

Finally, cost cannot be ignored. According to the Pentagon, the war in Afghanistan is costing $45 billion per year. A report published last year by the Center for Strategic and International Studies concluded 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 when other war-related costs are included.

No wonder a recent survey found that 57 percent of Americans, including 69 percent of military veterans, said they would support a decision by the president to remove all troops from Afghanistan.

It would be easy to dismiss President Trump’s decision to begin withdrawing from Afghanistan as simply a political decision in which Trump is making good on a campaign promise. The reality is that it reflects what Americans want because they understand that a continued U.S. military presence Afghanistan no longer is in our national security interest— it’s a conflict we don’t need to fight, can’t win, and can’t afford.