Forty years ago, Saigon fell to communist North Vietnam. Images of terrified South Vietnamese clambering to the roof of the U.S. embassy, and Vietnamese helicopter pilots ferrying them to ships and then pushing the helicopters overboard to make room for more refugees are still heart-wrenching.

Vietnam represented a change in the American security dynamic for the protection of friends and the defeat of adversaries, but the application of useful policy lessons is hard to find.

WWII and the Korean War had required the United States to leave substantial parts of its military in place either to consolidate victory or prevent the erosion of an armistice. In the case of Vietnam, however, the U.S. armed and trained the South Vietnamese military (ARVN) and then left it to the field. The policy was called “Vietnamization.” The question should have been asked, “What is the staying power of an army when its enemy consists of its brothers and cousins -- that is to say, when it is fighting a civil war -- in the absence of U.S. support on the ground?” And, the corollary, “Particularly when its brothers and cousins are supported by outside powers?”

The policy of arming/training forces to maintain control and friendly governments carried over to other countries. The Palestinian Security Forces, Mali, Yemen, Pakistan, El Salvador, Colombia, the Philippines, and, of course, Iraq and Afghanistan. Some have worked out better than others. The Iraqification and Afghanization of America’s 21st century wars, where U.S. withdrawal, or even the plan for withdrawal, encouraged internal forces -- supported by others -- have not been encouraging.

Vietnam also marked the first occasion the United States accepted a paper agreement with little American control over future events should the adversary decide to change or ignore the terms of the treaty. Le Duc Tho and Henry Kissinger were awarded the Nobel Prize for the 1973 Paris Treaty, which assured South Vietnam of a continued American interest and American support in the case of a North Vietnamese attack. But it wasn’t actually a treaty, wasn’t ratified by Congress, and did not outlast the president who made it (proving the relevance of Sen. Tom Cotton’s letter to the Ayatollahs). Congress, in fact, denied the White House funds it sought to help the South. President Ford announced in January 1975, as the North Vietnamese were coming south across the border, that the U.S. would not support South Vietnam militarily. Le Duc Tho -- who declined to accept his Nobel Prize -- had lied and there was nothing the U.S. was prepared to do about it.     

Bloody and ugly, Vietnam was nevertheless simple for the Americans. When domestic public opinion would no longer tolerate casualties in a war that appeared unwinnable, the U.S. took its soldiers and left, averting its eyes from the wreckage it left behind. The war did not follow us home -- except for the thousands of “boat people” who fled on rickety craft, braving seas, weather, and the studied indifference of the world. North Vietnam did not recruit suicide bombers or plan for worldwide domination.

In part as a reaction to the grinding of the Vietnam War, the 1991 “Powell Doctrine” posited that the U.S. should use overwhelming force -- heavy on ground troops -- to achieve discrete U.S. national security objectives only in a situation with widespread public support. Conveniently, that was the case in the First Gulf War, reversing Iraq’s invasion of Kuwait. But then-Secretary Powell forgot to posit a discrete end to the war (see Every War Must End), and American forces remained bogged down in the enforcement of “no fly zones” for 12 years until the invasion of Iraq itself in 2003.

The Iraq War used the “arm and train the locals” syndrome, which -- like Vietnam -- was successful as long as the U.S. was there to be a partner. The “surge” worked because Iraqi forces had Marines with them, and sectarian fighting was reduced because neither Sunnis nor Shiites were willing to engage in a civil war with the Marines standing by. President Obama, calling Americans “war weary,” pronounced Iraq “stable and self-reliant” as he pulled American troops out in 2011. The decline was swift and ugly -- and necessitated the return of Americans in uniform; several thousand to date, and an effort to build a new and improved Iraqi military.

Afghanistan follows the pattern. President Obama, saying, “I was elected to end wars, not start them,” announced the end of the American military presence in Afghanistan years in advance. As the date grows closer, the Afghan National Army is not prepared to defend the country, according to many U.S. experts. The Taliban is increasing its attacks on the Kabul government, which is trying to end the war by engaging the Taliban in “peace talks.”

Libya? There was no American plan in Libya beyond the ouster of the Gaddafi government. Chaos reigns to the benefit of al Qaeda.

Mali? The U.S. armed and trained the Malian military, which supported a generally democratic government. However, the military overthrew the government, the U.S. cut off its support and al Qaeda-supported Tuareg rebels outfitted with Libyan arms descended on the country. Only French intervention prevented an al Qaeda victory in Bamako.

Yemen? The great success touted by the Obama Administration for the policy of arming and training Yemeni forces and conducting drone strikes in its territory appears to have blinded the administration and the Yemeni government to the Houthi threat until it was too late.

Unlike Vietnam, in these current cases -- Iraq, Afghanistan, Libya, Mali and Yemen -- plus Syria where the U.S. is doing on-again-off-again training of “moderate rebels,” the adversary is or is linked to forces that very much want to attack American interests at home and abroad.

There is no nostalgia for Vietnam; 58,000 Americans lost their lives there and countless thousands were wounded and traumatized. But 40 years after, it is clear that pulling out and leaving the Vietnamese to their fate was almost cost-free for us, whatever costs it imposed on the Vietnamese.

That no longer applies. We cannot withdraw from the war and we appear to have no strategy for winning it.