CUMBERLAND — April 28 marked the 40th anniversary of the fall of South Vietnam. Last week my wife, Sally, and I attended a public MPBN screening of the Oscar-winning documentary “Last Days in Vietnam,” which aired this week nationwide on PBS’ “American Experience.”

It is sobering stuff, particularly for this Navy Vietnam vet, who served a tour of duty on a destroyer in the Tonkin Gulf. Watching the documentary, I was struck by the scale of the betrayal that marked our last days in Vietnam.

In convincing the South Vietnamese to sign the Paris Peace Accords in 1973, Secretary of State Henry Kissinger, the architect of the agreement, assured the South Vietnamese that we would stand by them if the North Vietnamese violated the cease-fire.

When the North Vietnamese did so in 1975, sending some 12 divisions southward, we did nothing. Congress even refused a modest presidential request of emergency aid. The South Vietnamese were left on their own, and they collapsed.

The documentary is a graphic display of that collapse, mainly from the perspective of American military and embassy personnel in Saigon. Many of these people are still alive, and their witness to the events makes for powerful television.

Emblematic of all that was good and bad about American policy in South Vietnam was Ambassador Graham Martin, a courageous and well-meaning diplomat caught in an impossible situation.

American intelligence documented the collapse of the South Vietnamese army and the imminent arrival of the North Vietnamese in Saigon.

However, Ambassador Martin felt he could not acknowledge publicly or privately that Saigon would fall. Therefore, he forbade evacuation planning until Saigon was virtually surrounded and the big American air base at Tan Son Nhut under attack. This action closed off all escape routes except those by helicopter from American carriers in the Tonkin Gulf – evacuation was left to the last two days and 19 Marine helicopters.

At this point, several thousand Americans and their families were still in Saigon, along with perhaps 100,000 to 200,000 South Vietnamese who were closely associated with America and likely to be on North Vietnam’s most-wanted list.

It was indeed an impossible situation. Americans and those South Vietnamese most closely associated with us had to gather immediately at the U.S. Embassy.

Rather bizarrely, the agreed-upon evacuation signal was to play Bing Crosby’s “White Christmas” over Armed Forces Radio. The significance of the song was not lost on most South Vietnamese, who were soon streaming toward the embassy by the thousands. All of this is masterfully captured in the documentary.

In spite of spineless leadership in Washington in which Kissinger was most complicit (though he was at pains in this documentary to place the blame elsewhere), many Americans in Saigon were heroic in their efforts to evacuate both Americans and as many South Vietnamese as possible.

The embassy guards, at great personal risk, stayed to the very end – assuring all those Vietnamese who had been given refuge in the embassy that they would find a way to get them out.

Though most of the approximately 5,000 Vietnamese in the embassy compound did get out, Washington’s courage faltered and led to a “presidential” order to cease the airlift and take the ambassador out, leaving some 400 Vietnamese still in the compound.

Much credit for what success there was in this 11th-hour evacuation goes to the Marine helicopter pilots, who flew repeat trips over 18 hours straight to try to get everyone out of the embassy. One of those Marine pilots is featured in the documentary. He looks like he still could be flying combat missions. He is a stirring example of all that we Americans can be – acting unselfishly and beyond the call of duty.

Another unsung hero is young Navy officer Rich Armitage (who would go on to an impressive diplomatic career). Armitage, an adviser to the Vietnamese navy, led a makeshift flotilla of ships crammed with refugees to safety in the Philippines – without approval from Washington. As Armitage put it, “I decided to seek forgiveness rather than approval.”

And so did South Vietnam fall. Hundreds of thousands of our allies were killed. Another million were imprisoned. Over the next several years, 1 million or more Vietnamese “boat people” would find their way to sanctuary in the United States.

Vietnam was a failure of U.S. policy of enormous proportions. Almost 60,000 U.S. troops killed, over 1 million Vietnamese. It was a loss that haunted U.S. military leadership for years.

This documentary forces us to remember an unfortunate legacy, and a lesson in humility that we still do not seem to fully understand.