As the United States leaves Afghanistan after 20 years of war, there can be little doubt that we lost the war — or to put it more gently, did not attain our objectives. In recent weeks, the Taliban have advanced across the north of the country. Bereft of U.S. support, the Afghan army and police have reportedly lost more than two dozen districts over the course of a month and are now fighting on the outskirts of key cities such as Kandahar and Mazar-e-Sharif. Senior U.S. officials have warned of a civil war, while intelligence reports are said to forecast the fall of the Afghan government — which the United States has worked to strengthen for two decades — within a year.

Why did we lose? I’ve been trying to answer that question for 12 years, starting in 2009 when I was a civilian officer in the far-off district of Garmser in Helmand Province. I continued to ponder the question in 2013 and 2014, when I served as political adviser to Gen. Joseph Dunford, commander of all U.S. forces in Afghanistan, and later as Dunford’s senior adviser when he was chairman of the Joint Chiefs of Staff. As I traveled the country with senior U.S. military commanders, I saw that in battle after battle, numerically superior and better-supplied soldiers and police were being defeated by poorly resourced and unexceptionally led Taliban — a dynamic certain to eventually doom the Afghan government unless the United States were to stay indefinitely.

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Carter Malkasian is the author of The American War in Afghanistan: A History. He served as a civilian advisor in Iraq and Afghanistan and was the senior advisor to General Joseph Dunford, Chairman of the Joint Chiefs of Staff, from 2015 to 2019.