This weekend, President Obama is visiting Vietnam. This should serve as a timely opportunity to contemplate the disturbing tendency of the United States to leave its allies in the lurch.

The United States pulled its troops out of South Vietnam in 1973 after the signing of the Paris Peace Accords, which allowed North Vietnam to maintain at least 150,000 of its forces in the South. President Nixon disingenuously assured South Vietnam’s President, Nguyen Van Thieu, that the lop-sided nature of the agreement didn’t matter because the U.S. would come to Saigon’s aid in the event of communist aggression. “If Hanoi fails to abide by the terms of this agreement it is my intention to take swift and severe retaliatory action,” Nixon secretly wrote to Thieu.

Well, we know what happened. North Vietnam never for a minute abided by the peace accords, and the United States never delivered the “swift and severe retaliatory action” that Nixon had promised. Worse than that, the U.S. cut aid to South Vietnam every year even as North Vietnam was rearming with the help of China and the Soviet Union. In the spring of 1975, North Vietnam launched its final offensive. South Vietnam crumbled within weeks while the U.S. did nothing to help.

Finally, at the last minute, the U.S. did step in to evacuate South Vietnamese who had worked closely with our forces and who would be at risk of retaliation. In all some 130,000 Vietnamese escaped, many on their own, but many more were left behind to face punishment as “enemies of the people.” Hundreds of thousands of South Vietnamese wound up in brutal “re-education camps.” Within a few years, hundreds of thousands of South Vietnamese took to the seas in leaky boats to seek freedom elsewhere. Many died in the attempt.

The people of Cambodia, abandoned by the United States at the same time in a paroxysm of isolationism, suffered even worse. Some two million died in the killing fields of Cambodia at the hands of the Khmer Rouge.

It is worth recalling this not-so-ancient history because of the very real danger that we will repeat these mistakes in Iraq and Afghanistan, where there is every reason to think the U.S. will tire of the struggle and abandon those who risked their necks to help us.

Ryan Crocker, former U.S. ambassador to Kabul and Baghdad, just penned an eloquent article calling on Congress to expand the number of Special Immigrant Visas that allow interpreters and other Afghans who worked with U.S. diplomatic, intelligence, and military agencies to move to the United States. Crocker writes: “There are 10,000 Afghans in the SIV application backlog. But the State Department has fewer than 4,000 visas remaining, which would leave more than 6,000 Afghans stranded in a country where their work for the United States means they are no longer safe.”

Yet far from increasing the allotment of Special Immigrant Visas, the House and Senate appear to be moving backward by restricting the criteria for immigration. This leaves Afghans like Naqueebullah Malikzada in a limbo that, in his case, has already lasted five years. As Stars and Stripes reported, he spent a decade working as an interpreter for U.S. military forces in Helmand province. His cache of photos attests to his daring deeds: “There he was in desert fatigues carrying an AK-47 with American soldiers, there he was manning a .50 caliber machine gun, there was the leg of a suicide bomber who carried out an attack that narrowly missed killing Malikzada.”

Now Malikzada is a hunted man, on the run within Afghanistan from the Taliban, who have repeatedly threatened to kill him. “If they catch you, they kill you,” he says.

It is a tragedy that the United States has turned its back on men like Malikzada who risked their lives to help our forces to accomplish their missions. But it is hardly a surprise. Just ask any South Vietnamese about what a fickle ally the U.S. can be. As Nguyen Van Thieu, the last president of South Vietnam, said: “It is so easy to be an enemy of the United States, but so difficult to be a friend.”