As we enter into a new decade, America has been at war in Afghanistan for almost twenty years — the longest conflict in U.S. history. What started in the immediate aftermath of the al Qaeda terrorist attacks on Sept. 11, 2001, as a counterterrorism mission has morphed into something more ambitious but less well defined and, ultimately, less successful.

The costs have been staggering — more than 2,000 U.S. service members have died in Afghanistan, over 15,000 have been injured, and — according to some estimates — U.S. spending may have totaled over $2 trillion.

Americans — from foreign policy elites in Washington to the general public — understandably want to see this chapter brought to a close. But history warns of the consequences when the United States leaves a conflict precipitously before the foundations for an acceptable political settlement are in place. In 1975, when the U.S. military withdrew fully from Vietnam, communist forces quickly overran a weak South Vietnamese government. If the Taliban were to sweep similarly to victory, harrowing human rights abuses could follow, and all our investments of blood and treasure could be undone.