It is hardly remarkable to say that success for the United States in Afghanistan was never likely. For years, experts, scholars, and officials have coalesced on a common set of causes for failure—local grievances, political factionalism, corruption, Pakistani safe havens, and a deep-seated Afghan resistance to occupation. All are notoriously intractable.

The difficulty of changing the course of the Afghan war begs the question why the United States didn’t just leave and curtail the expense. “The failure of American leaders—civilians and generals through three administrations, from the Pentagon to the State Department to Congress and the White House—to develop and pursue a strategy to end the war ought to be studied for generations,” read the New York Times editorial page in February 2019.

I have met few Americans who disagree that after September 11, 2001 we were right to go in to Afghanistan, hunt down al-Qaeda, and overthrow the Taliban. To the end of 2019, Gallup and other credible public opinion polls found that a majority of Americans never believed that the U.S. decision to invade Afghanistan was a mistake. Yet, in my anecdotal experience, most Americans also assume we should have left some time after that.

So why didn’t we leave? To some extent, we didn’t understand Afghanistan. The lightning toppling of the Taliban regime in 2001 defused early concerns in the Bush administration that Afghanistan would become a quagmire. The success of the surge in Iraq a few years later encouraged U.S. military and civilian leaders in the Obama administration to think that the Afghan democracy might be enabled to stand on its own.

Another argument is that U.S. generals and military officers did not want to leave. There is something to this, though perhaps less than meets the eye. At times, high-ranking military officers put forth plans to reinforce Afghanistan, such as for the 2009 surge or the Trump administration’s 2017 South Asia strategy. At other times, they requested that withdrawal timelines be extended or removed, such as when Taliban offensives endangered the democratic government in 2015 and 2016.

Such actions, many of which were the natural and correct part of the national security decision-making process, raised the political costs for any president seeking to withdraw. If things went bad, the president could be accused of acting against the advice of the military.

But when trying to understand why we didn’t leave, the heart of the matter really comes down to terrorism and its domestic political implications. The overriding question in all policy debates was what level of terrorist threat would emerge if the democratic government in Afghanistan fell. Indeed, generals would have had no leg to stand on without the very real and palpable terrorist threat.

The Sept.11 attacks ignited a fear that presidents could not ignore. Previously a minor irritation, terrorism transformed into a real threat to the United States, with the potential to involve chemical, biological, or nuclear weapons.