Frank Snepp is a Peabody-award winning investigative journalist and the author of two CIA memoirs.

Thirty-eight years ago last week, I was among the last CIA officers to be choppered off the U.S. Embassy roof in Saigon as the North Vietnamese took the country. Just two years before that chaotic rush for the exits, the Nixon administration had withdrawn the last American troops from the war zone and had declared indigenous forces strong enough, and the government reliable enough, to withstand whatever the enemy might throw into the fray after U.S. forces were gone.

That's the same story we told ourselves in Iraq when we pulled out of that country in 2011. And today, as American troops are being drawn down in Afghanistan, we're hearing variations on the same claims once again. Yet security remains so fragile in both Iraq and Afghanistan, it is impossible not to worry that we are deluding ourselves and that we failed to learn the most important lessons of Vietnam.

One major ingredient of both the Afghanistan and Iraqi experiments was the use of American dollars to buy off insurgents, wean them from their Al Qaeda or Taliban suitors and win the indulgence, however grudging, of the leadership in Kabul or Baghdad. Such payments may help ensure a lull in the violence to allow U.S. forces to withdraw. But the enduring fallacy of such tactics was made clear in Vietnam....