The dulcet tones of “White Christmas” that crackled over Armed Forces Radio airwaves on April 29, 1975, failed to spread cheer across sunbaked Saigon. Instead, the broadcast of the holiday standard after the announcement that “the temperature in Saigon is 105 degrees and rising” instilled fear and panic in all who recognized the coded signal to begin an immediate evacuation of all Americans from Vietnam.

Although the United States had withdrawn its combat forces from Vietnam after the signing of the Paris Peace Accords in 1973, approximately 5,000 Americans—including diplomats, marine guards, contractors and Central Intelligence Agency employees—remained. President Richard Nixon had secretly promised South Vietnam that the United States would “respond with full force” if North Vietnam violated the peace treaty. However, after the Watergate scandal forced Nixon to resign, the North Vietnamese Army felt emboldened to launch a major offensive in March 1975.

“From Hanoi’s point of view, the turmoil leading up to and including Nixon’s resignation was an opportunity to take advantage of a distracted United States,” says Tom Clavin, co-author of Last Men Out: The True Story of America's Heroic Final Hours in Vietnam. “North Vietnam never intended to abide by the 1973 agreement—its ultimate mission was to unify the country—but the political crisis in America allowed them to move up their timetable.”

After winning a decisive battle at Ban Me Thuot and capturing the central highlands, the North Vietnamese Army swept south and captured the cities of Quang Tri and Hue with little resistance and no American response. The fall of Da Nang, South Vietnam’s second-largest city, on March 29 unleashed a furious exodus that included desperate residents clinging to the rear staircase and landing gear of a World Airways plane and falling to their deaths as it took flight. After watching news coverage of the incident, President Gerald Ford confided to an aide, “It’s time to pull the plug. Vietnam is gone.”

With little American appetite for re-engaging in the Vietnam War, Congress rejected Ford’s request for $722 million to aid South Vietnam. When communist forces seized Xuan Loc on April 21, South Vietnamese President Nguyen Van Thieu resigned and fled the country as 150,000 enemy troops stood on the footsteps of Saigon.

Inside the South Vietnamese capital, U.S. ambassador Graham Martin rebuffed repeated calls to even consider an evacuation, let alone execute one. Martin, who had been ill for months, was fearful of inciting panic in the city and determined to fulfill the mandate given to him by Nixon upon his appointment two years earlier to preserve South Vietnam’s existence.

“Like the country he was ambassador to, Martin was barely functioning in April 1975,” Clavin says. “The physical and emotional exhaustion of Martin affected his decision-making. Even the most robust ambassador would have been affected by the tremendous strain of representing a failed U.S. policy and walls crashing down all around him.”

Early on the morning of April 29, North Vietnamese troops shelled Saigon’s Tan Son Nhut Air Base, killing two U.S. Marines guarding the defense attaché office compound. Corporal Charles McMahon and Lance Corporal Darwin Judge were the last of approximately 58,000 American servicemen killed in action in the Vietnam War. After surveying the air base damage, Martin conceded the time had come to leave Saigon, but with sea lanes blocked and commercial and military aircraft unable to land, the ambassador’s delays forced the United States into its option of last resort—a helicopter airlift.

Once the “White Christmas” signal was given to launch the exodus, codenamed Operation Frequent Wind, Americans and their Vietnamese allies assembled at pre-arranged locations to board buses and helicopters to the defense attaché office compound where larger helicopters ferried them to U.S. Navy ships 40 miles away in the South China Sea.

Approximately 5,000 escaped from the defense attaché office compound until enemy fire forced the American embassy to become the sole departure point. While plans called for the extraction of only Americans, Martin insisted that Vietnamese government and military officials and support staff also be evacuated.

“Looking past his mistakes, Martin was a good man,” Clavin says. “Martin really cared about the native population, and like many others he expected a bloodbath once the North Vietnamese entered the city. With everything else failing, at least he could save some lives before it was too late.”

While approximately 10,000 people clamored outside the embassy gates, marine guards faced the unenviable task of deciding who would be saved and who would be left behind. Through the day and into the night, helicopters landed at 10-minute intervals on the embassy roof and in an adjacent parking lot.

Meanwhile, South Vietnamese air force pilots commandeered helicopters, loaded their families on board and landed on the decks of American ships. So many South Vietnamese helicopters besieged the fleet that crews were forced to push helicopters into the sea in order to make room for others to land.

Martin repeatedly refused to leave his post to ensure as many people as possible were airlifted. In spite of his wish, however, the Americans simply couldn’t take everyone amassed at the embassy. At 3:30 a.m. Ford ordered Martin out of the embassy and stipulated that only Americans would be evacuated on the remaining flights. Ninety minutes later, Martin departed after being handed the folded embassy flag.

The last marines to vacate the embassy departed just after dawn on April 30, leaving behind hundreds of Vietnamese. As the helicopter carrying the marines vanished from view so did the American presence in Vietnam. (An iconic photograph of Vietnamese evacuees climbing up a rickety wooden staircase to a helicopter on an apartment building roof the previous day is often misremembered as the last helicopter to leave the American embassy.) With some pilots flying for 19 hours straight, the American military had carried out an incredible evacuation of 7,000 people, including 5,500 Vietnamese, in less than 24 hours.

Hours after the departure of the last helicopter from the embassy, North Vietnamese tanks smashed through the gates of the Independence Palace. General Duong Van Minh, who succeeded Thieu as president, offered an unconditional surrender, officially ending the two-decade-long Vietnam War. The new regime rechristened Saigon as Ho Chi Minh City to honor the late North Vietnamese president.