Since publishing a book on America’s age of unwinnable conflicts a couple of weeks ago, I’ve been asked hundreds of questions about topics ranging from ISIS to Ukraine, from the military-industrial complex to reinstating the draft.

But I can’t recall a single person asking me about the war in Afghanistan.

This is surprising given that the U.S. military campaign in Afghanistan is not only ongoing, but has cost nearly 2,500 American lives and hundreds of billions of dollars, is teetering somewhere between stalemate and loss, and represents the latest chapter in America’s saga of intractable conflicts.

It seems as if Americans have signed onto a pact of forgetting: a collective effort to expunge all memory of the war in Afghanistan.

Afghanistan was once the good war. Seeking righteous vengeance for 9/11, nearly 90 percent of the American public initially backed a crusade to topple the Taliban regime in Kabul and purge the country of al-Qaeda. For years, Barack Obama described Afghanistan as the center of gravity in the struggle against international terrorism. In 2009, Obama surged U.S. forces in Afghanistan to nearly 100,000 troops. With money no object, U.S. officials tried to win over Afghans through initiatives like an Afghan version of Sesame Street called Sesame Garden (unfortunately, the Count character had to be cut because Afghan kids weren’t familiar with Dracula and were confused by his fangs).

And then the good war turned bad. Obama became disillusioned with the lack of progress in Afghanistan and looked for the quickest possible exit. By 2011, then-Secretary of Defense Robert Gates had concluded that for Obama, “it’s all about getting out.” The president’s exhaustion mirrored the national mood. After a decade and a half of conflict, the public has moved beyond war weariness into a kind of numbing amnesia.

Americans, it seems, have agreed to a pact of forgetting. The term dates back to the death of Spanish dictator Francisco Franco in the 1970s, when Spaniards decided the best way to transition to democracy was el pacto del olvido, or a covenant to disremember the wounds of civil war and autocracy.

Americans are similarly blocking out the painful experience in Afghanistan. It takes a concerted effort—whether conscious or subconscious—to not think about a war where thousands of fellow citizens have died. Granted, the erasure is incomplete. The war flickers at the edges of people’s consciousness. But the mind rebels against giving the conflict any serious contemplation. Raising the topic of Afghanistan these days is like mentioning mortality. There’s a profound desire to change the subject. The popular narrative was once about saving Afghans. Now the focus is on getting American soldiers home, and Afghans have disappeared from the story.

Opinion polls suggest that public support for the war dropped to record lows in 2013. And revealingly, there have barely been any polls on Afghanistan in 2015. The media has also turned away from the war. To pick on the paper of record, the number of New York Times articles mentioning “Afghanistan” roughly halved during the first five months of 2015 compared with the same period in 2012.

Americans are instead debating whether to start a full-scale war against ISIS, a Wahhabi Islamist insurgent group in a strategic region bordering Iran. Few seem to have noticed that the United States is already at war with a Wahhabi Islamist insurgent group in a strategic region bordering Iran: the Taliban in Afghanistan.

Difficult wars often end in a pact of forgetting. When Saigon fell to Communist forces in 1975, journalist Martha Gellhorn said, “consensual amnesia was the American reaction, an almost instant reaction, to the Vietnam War.” In part, forgetting Vietnam was a subconscious effort to avoid pain. But it was also a very deliberate attempt to cast off the war. The U.S. Army actually destroyed its notes on counterinsurgency in a bid to consign the whole experience to oblivion.

In the case of Afghanistan, the lurch from fervor to amnesia has created a mismatch between U.S. interests and enthusiasm. America’s core goals in Afghanistan haven’t changed much over time: stopping international terrorists from using the country as a base and avoiding the destabilization of neighboring Pakistan. The American commitment to the campaign, however, has risen and crashed like a wave.

During Obama’s first term, the scale of U.S. military intervention in Afghanistan arguably outweighed American interests. Troop reinforcements were necessary in 2009 to check the Taliban’s advance. But the administration’s rapid tripling of U.S. forces was out of step with the limited nature of the president’s key objectives in the country. The White House deployed 100,000 troops in a country where perhaps fewer than 100 al-Qaeda members remained.

But now that this wave of U.S. commitment has dissipated, Americans face the opposite danger: forgetting the war and resisting the fairly modest investment in aid to Afghanistan that could make a meaningful difference in the outcome of the conflict. The United States and its allies, for instance, have promised to fund Afghan security forces through 2017—a commitment that should be extended until the end of the decade. The cost of doing so, roughly $5 billion per year, is a small fraction of the $100 billion that the United States poured into Afghanistan in 2011. There are currently 10,000 U.S. troops in Afghanistan on a training and counterterrorism mission. Obama has promised to remove them by the end of 2016—a date seemingly chosen so the president can say he concluded the war before leaving office. It would be better for a few thousand troops to remain and train Afghans after 2016, even if it complicates Obama’s preferred narrative. Without such investment, what are the prospects for Ashraf Ghani and Abdullah Abdullah’s experiment in unified government in Kabul? Can the Afghan security forces survive record casualties?

Amnesia can be an effective coping strategy. Nietzsche said it was useful “to close the doors and windows of consciousness for a time.” Putting difficult events out of mind is “like a doorkeeper, a preserver of psychic order, repose, and etiquette.”

But expunging a negative experience of war may prevent an open and honest debate about how to salvage the campaign—and absorb its lessons before committing the same mistakes again. We forget so we can ease the pain. But the pain is how we learn.