During the Vietnam War, negotiators in Geneva agreed that Laos would remain neutral. But because the United States feared the spread of communism, the C.I.A. directed a covert operation in Laos known as the Secret War. It recruited Vang Pao, a charismatic, widely respected general, along with tens of thousands of Hmong boys and men, as fighters. For a decade and a half, Gen. Vang Pao and his Hmong guerrillas fought alongside the Americans.

On that chaotic morning, the Secret War came to a jarring end. After a quick extraction followed by decades in exile, General Vang Pao, who died in 2011, never saw his homeland again. Many of his top officials had been evacuated in previous flights, but thousands of people were abandoned on that airstrip, and even more were left behind elsewhere in Laos. Long Tieng, once a stronghold symbolic of the American occupation, was now in ruins.

This month marks 40 years since the fall of Long Tieng and the start of the Hmong exodus from Laos. It marks the desertion of a people left to fend for themselves, with nothing to rebuild their lives. It marks the treachery of the United States government, which went into an unknown country, waged years of war, and then dropped everything in a moment’s notice. It marks a landscape devastated by conflict, with American warplanes dropping enough ordnance on Laos to equal a planeload of bombs every eight minutes, 24 hours a day, for nine years, according to Legacies of War, an organization that works to raise awareness about unexploded ordnance.