Part of a series we're calling "40 Years Later: A Return To Vietnam," on the 40th anniversary of the fall of Saigon and the final evacuation of U.S. troops.

BOSTON — Amid preparations for Reunification Day this week, Ho Chi Minh City, as it's now known, stands in stark contrast to the images the world saw 40 years ago, when the North Vietnamese closed in on Saigon.

After signing the Paris Peace Accords with North Vietnam in 1973, the U.S. began drawing down its troops.

By the end of April 1975, the U.S. troop presence in South Vietnam — which had been over half a million in 1969 — had dwindled to several dozen Marine Security Guards protecting the U.S. Embassy in Saigon, plus other U.S. personnel stationed at an airbase a few miles away.

Of those Marines, four were from Massachusetts. A fifth, 21-year-old Charles McMahon, from Woburn, would be one of the last two U.S. troops to die in Vietnam — just days after his arrival.

'Nightmarish'

Forty years ago, Saigon had descended into chaos.

In those last days of April 1975, the final evacuation of U.S. troops, and the South Vietnamese who supported them, was underway. This was as around 100,000 North Vietnamese troops closed in on Saigon, using rocket and artillery fire to disrupt air operations at Tan Son Nhut airbase.

While thousands of American military personnel and citizens were evacuating in helicopters from the embassy roof, thousands of others gathered outside the embassy walls, hoping to make their way out.

Americans and Vietnamese run for a U.S. Marine helicopter in Saigon while evacuating the city on April 29, 1975. (AP)

Inside the embassy compound was Marine Bill Newell, of Hopkinton. He described the scene as "nightmarish."

"It wasn't even in the plans to ever evacuate from the embassy and now we're in a position where we've got people who are scared to death all around us, there's forces out in the streets with weapons," he recalled recently. "People are trying to get over our walls and now we've got helicopter landings on the rooftop and in the parking lot."

When the embassy gate was ordered locked, panic grew in the crowd outside.

Six-foot-four Marine John Ghilain, who’s now a Medford police officer, boosted his sergeant up on top of the nine-foot main gate to help haul people over.

"Literally pulling bodies over the wall," he said in a recent interview. "We were told anybody that showed an American passport, get him inside the compound, get him out of here."

Also in the crowd outside was a British television crew. From their broadcast: "The embassy gates were closed and we, like the frightened Vietnamese and their families, had to fight and claw our way up, and we did claw and we did fight."

The TV crew got in, and was taken by helicopter to one of the many U.S. ships offshore receiving evacuees. Thousands were evacuated that way.

On April 29, U.S. Ambassador Graham Martin ordered a motorcade to observe the evacuation operation at Tan Son Nhut.

Paul Gozkit, a Marine from Chicopee, was the ambassador’s driver. As they arrived, the airbase was under rocket attack.

U.S. Ambassador to South Vietnam Graham Martin speaks to the press aboard the USS Blue Ridge after being evacuated from the U.S. Embassy in Saigon on April 30, 1975. (AP)

"I stood outside and I mean I actually saw a rocket go by, it was flying by and I said, 'Oh, this is real stuff,' " he said. "So I was kind of like under the car a little bit. The rockets were lobbing over the facility and it ... there was a lot of flames and fires."

When they got back to the embassy, the ambassador ordered Gozkit to immediately evacuate his wife, Dorothy Martin.

"And I had my radio, I had a Swedish K submachine gun and my revolver, and that's it," Gozkit remembered. "The clothes on my back, that was it."

Gozkit and the ambassador’s wife were helicoptered out to the USS Blue Ridge, the command ship of the 7th Fleet. He was the first of those last four Massachusetts Marines to leave Vietnam.

Evacuations continued through the day and into the night. Early the next morning, April 30, the Marines received orders: No more civilian evacuees. They backed their way into the building, locked it up, and went to the roof.

'Our Alamo'

But the flights, suddenly, stopped coming.

Marine Carlos Silva, a New Bedford native, was on the roof. It was his 23rd birthday.

"Well it’s hectic, cause by that time the Vietnamese knew that no more choppers were coming in, they’d figured it out," Silva said. "At six o’clock the last chopper left. We were standing and we're wondering, 'Where’s the choppers? Where are they? Did they forget us or what?' "

Newell called waiting in those hours "excruciating."

"At that point I was really feeling the fear because as I was laying on the roof I was thinking about, 'All they need to do is lob an artillery shell in here and we're done,' " he said.

Ghilain remembers what the Marine commander, Maj. James Kean, said to the group.

"And he said, 'Well, we're Marines. We're going to defend this embassy. It's our Alamo,' " Ghilain said. "And that's when we all looked at one another and went, 'Whoa. All right, we're in serious trouble here.' "

A U.S. Marine helicopter lifts off from atop the U.S. Embassy during the evacuation of Saigon on April 30, 1975. (AP)

But the helicopters did come and the final U.S. troops to leave Vietnam, including these three Massachusetts Marines — Newell, Ghilain and Silva — climbed on to the next-to-the-last U.S. helicopter out of Saigon.

Newell recalled the time: 07:35, April 30, 1975.

"And then we were flying out over the South China Sea," he said. "I just remember, what a gray day it was. It was just very gray."

Newell and Ghilain are in a group of Marines back in Vietnam this week to pay tribute to the last two Marines killed in the war, including Charlie McMahon from Woburn.

For Ghilain and Newell, it's their first time back in Vietnam back since they boarded the helicopter on the roof of the embassy 40 years ago.

"My primary reason is, uh, it's an emotional thing, too, so," Ghilain said in an interview back at home, his voice cracking. "To go back, honor the two guys that we lost. I'm hoping that this trip puts this to bed for good."