Unlike the wars in Vietnam and Iraq, the US invasion of Afghanistan in 2001 had near-unanimous public support. At first, the goals were straightforward and clear: defeat al-Qaeda and prevent a repeat of 9/11. Yet soon after the United States and its allies removed the Taliban from power, the mission veered off course and US officials lost sight of their original objectives.

Distracted by the war in Iraq, the US military become mired in an unwinnable guerrilla conflict in a country it did not understand. But no president wanted to admit failure, especially in a war that began as a just cause. Instead, the Bush, Obama, and Trump administrations sent more and more troops to Afghanistan and repeatedly said they were making progress, even though they knew there was no realistic prospect for an outright victory.