As NATO leaders prepare to gather in Wales this week, they are being asked to consider initiatives that range from possible new sanctions against Russia to the creation of a rapid reaction force to defend Eastern European members to increases in defense spending to a stronger response to the Islamic State.

All of these ideas have merit. But one idea that lacks merit yet could be tempting for leaders to endorse is a formal alliance reaffirmation of President Barack Obama’s plan to remove all U.S. combat units from Afghanistan by the end of his presidency in late 2016. NATO should not endorse this policy to preserve flexibility for the future.

As one of us (Riedel) has recently written in a new book about the war in Afghanistan in the 1980s, troubles won’t end in that uneasy part of the world just because America decides to leave. Positive accomplishments are often fragile, and could be reversed quickly if we abandon our interests and commitments too soon. One of the most important achievements by Western countries as well as Pakistan and Persian Gulf allies during the Cold War was the defeat of Soviet forces in Afghanistan in a conflict that helped seal the end of not just that Cold War but also the Soviet Union itself. But those gains were partly squandered by America’s disinterest in Afghanistan after 1989, which contributed to the rise of the Taliban, the sanctuary established by al Qaeda in the 1990s, and the 9/11 attacks of 2001.

More recently, there has been tremendous progress in Afghanistan, despite all the frustrations. The Taliban was driven out of power in late 2001, and in the dozen years that followed, much of al Qaeda’s leadership was decimated, largely by U.S. drone strikes in the tribal areas of Pakistan, where many top al Qaeda leaders had fled. Al Qaeda leader Osama bin Laden was killed, and Afghanistan was able to build up an army and police force that–so far, at least–has fended off the Taliban even as NATO has dramatically reduced its role in the country.

This year, U.S. forces have been reduced by two-thirds from their peak, and American casualties have decreased by nearly three-fourths as Afghan security personnel take the lead on more than 95% of all operations. At the end of this year, U.S. forces will dip slightly below 10,000, just 10% of their earlier high. Other NATO forces will number several thousand. Afghan forces will total 350,000 at the end of the year. In 2013 alone, some 4,700 Afghan troops were killed. Although it is clearly regrettable that casualties remain so high and the Taliban remains so resilient, it is impressive how tenaciously these brave Afghan patriots are defending their country.

President Obama now risks pulling the plug on all this progress. While he has shown remarkable resilience so far, his current plan would downsize U.S. forces to 9,800 by Jan. 1, halve that number by early 2016 and withdraw all combat units by the end of that year. Because NATO has an “in together, out together” strategy, and because many European allies depend on America for certain key forces and capabilities, it is nearly certain that other alliance members would withdraw their forces, too.

He should reassess. Afghan forces may still need certain help after 2016 in certain areas such as air power. That would not require a large U.S. force, and a modest enduring presence could be very helpful in protecting the investment we have made in what has become easily America’s longest war. Annual costs and casualties would be perhaps 5% what they were at the peak of our fighting early in the Obama years–a real price, but a manageable one given the alternatives.

And it is not just about helping the Afghans. We need bases in Afghanistan after 2016 to help–and defend–ourselves. Two to three bases with drones and special forces, able to strike al Qaeda targets or related extremists who may remain in western Pakistan or eastern and southern Afghanistan, would help American national security. There are no other good places on land or at sea from which we can carry out such attacks promptly and effectively, given the region’s geography.

Mr. Obama would do his successor no favor by depriving him or her of these kinds of national assets–and he should remain open, therefore, to the possibility of changing his zero option plan for 2016 somewhere down the road. The NATO summit should accord him, and other alliance leaders, that flexibility.

Today, Iraq is a fresh reminder that wars don’t end when America leaves. President Obama won’t end the Afghan war in 2016, and he may leave an explosive mix behind if he exits as precipitously as America left Afghanistan and Pakistan in 1990.

He should rethink his announcement that all American forces will leave by December 2016 and let conditions on the ground determine what U.S. military and intelligence capabilities will remain in 2017 and beyond. His successor should not be boxed in by a decision made in mid-2014 about what Afghanistan needs in 2017. To do so would ignore one of the most important lessons of the secret war of the 1980s.

The danger in Afghanistan and Pakistan tomorrow is the same as the danger in Iraq today. Without a unilateral counterterrorism capability in Afghanistan, al Qaeda may regenerate in South Asia as quickly as it has regenerated in Iraq. In fact, al Qaeda is likely to regenerate faster in Pakistan given the allies it has there once drone operations from bases in Afghanistan cease. No U.S. military force in Afghanistan means no unilateral counterterrorism capability. The President should keep his (and his successor’s) options open for the future in Afghanistan, not close them prematurely or precipitously.

This opinion originally appeared at
The Wall Street Journal
‘s Washington Wire