A South Vietnamese tank loaded with troops rumbles past a destroyed hamlet on Highway 22 following record fighting in the area about 60 miles northwest of Saigon. Officials said government reinforcements were rushed to Hieu Thien in an effort to retake outposts and villages that fell to the North Vietnamese, March 17, 1975.

Nowhere to go and nothing to do, South Vietnamese refugees from Hue and the northern provinces pause on the dock waiting for the government to relocate them to the central coastal area at Da Nang in Vietnam, March 28, 1975.

South Vietnamese Marines leap in panic aboard a cutter from an LST in Danang Harbor in Da Nang, Vietnam, April 1, 1975 as they are evacuated from the city, shortly before its fall to the Viet Cong and North Vietnamese. Cutters in turn hauled them south to Cam Ranh Bay.

A U.S. civilian pilot in the aircraft doorway tries to maintain order as panicking South Vietnamese civilians scramble to get aboard during evacuation of Nha Trang, April 1, 1975. Thousands of civilians and South Vietnamese soldiers fought for space on the aircraft to Saigon as communist forces advanced following the fall of Qui Nhon, to the north.

A South Vietnamese father carries his son and a bag of household possessions as he leaves his village near Trang Bom on Route 1 northwest of Saigon, April 23, 1975. The area was becoming politically and militarily unstable as communist forces advanced, just days before the fall of Saigon.

U.S. President Gerald Ford, Secretary of State Henry Kissinger and Vice President Nelson Rockefeller discuss the evacuation of Saigon at the White House in Washington, D.C., April 28, 1975.

U.S. President Gerald Ford (C) and U.S. Secretary of State Henry Kissinger(L) are briefed by National Security Adviser Brent Scowcroft (R) on the evacuation of Saigon, April 28, 1975 at the White House in Washington, DC.

Two Vietnamese women mourn their relatives April 29, 1975 at Bien Hoa military cemetery as the U.S. started to evacuate their embassy in Saigon, ending the U.S.'s 15-year involvement in Vietnam.

Americans and Vietnamese run for a U.S. Marine helicopter in Saigon during the evacuation of the city, April 29, 1975.

Vietnamese civilians wait in front of the U.S. embassy in Saigon for visas to live in the United States two days before the fall of the city to North Vietnam communist forces, April 28, 1975,

Mobs of Vietnamese people scale the wall of the U.S. Embassy in Saigon, Vietnam, trying to get to the helicopter pickup zone, just before the end of the Vietnam War, April 29, 1975.

In this video still, a group of people scale a stairway to a CIA helicopter on a rooftop in Saigon, April 29, 1075.

The black and white photo of this event by Dutch photojournalist Hugh Van Es became an iconic image of the war, symbolizing a desperate U.S. withdrawal from Vietnam.

The location of this evacuation has often been identified as the rooftop of the U.S. embassy, but in fact it wasn't. The people in the photo were on a rooftop of an apartment building housing CIA officials and their families.

U.S. Navy personnel aboard the USS Blue Ridge push a helicopter into the sea off the coast of Vietnam in order to make room for more evacuation flights from Saigon, April 29, 1975.

An injured South Vietnamese war veteran limps away on a crutch with food looted from abandoned U.S. installations after evacuation of Saigon, April 29, 1975.

The air evacuation of siege-stricken Vietnamese from Saigon to the U.S. was conducted after the Babylift operation. Vietnamese military and civilians await their C-41 journey during a stopover, in Utapo RTNAF, Thailand, April 29,1975.

A South Vietnamese mother and her three children crouch on the deck of an amphibious command ship being plucked out of Saigon by U.S. Marine helicopters in Vietnam, April 29, 1975.

The war ended on April 30, 1975, with the fall of Saigon, now known as Ho Chi Minh City, to communist troops from the north.

People try to scale the 14-foot wall of the U.S. Embassy in Saigon, trying to reach evacuation helicopters, as the last of the Americans depart from Vietnam, April 29, 1975.

The war ended April 30, 1975, with the fall of Saigon, now known as Ho Chi Minh City, to communist troops from the north. Neal Ulevich/AP Photo

South Vietnamese civilians try to scale the 14-foot wall of the U.S. embassy in Saigon, trying to reach evacuation helicopters as the last Americans departed from Vietnam, April 29, 1975.

More than two bitter decades of war in Vietnam ended with the last days of April 1975.

A North Vietnamese communist tank driving through the main gate of the presidential palace of the U.S.-backed South Vietnam regime as the city falls into the hands of communist troops April 30, 1975.

A line of captured U.S.-backed South Vietnamese Army soldiers are escorted by Vietnamese communist soldiers as they walk on a Saigon street after the city fell into the hands of the communist troops on the same day, April 30, 1975.

Tanks of the North Vietnamese Army (NVA) forces take over the South Vietnamese presidential palace April 30, 1975, South Vietnamese government's last stronghold.

North Vietnamese soldiers sit on top of a tank posted in front of the presidential palace of the U.S.-backed regime as hundreds of Saigon residents gather around after the city falls to communist troops on the same day, April 30, 1975.

A communist soldier at the Independence Palace in Saigon, April 30, 1975.

April 30, 2015 marks the 40th anniversary of the fall of Saigon to North Vietnamese forces and the subsequent toppling of the U.S.-supported southern regime.

A soldier perched on a tank of the North Vietnamese Army (NVA) in Saigon, as the city falls into the hands of communist troops, April 30, 1975.

A soldier of the National Liberation Forces (NLF) of North Vietnam surveys the National Assembly building in Saigon, after the fall of the city, April 30, 1975.

Duong Van Minh (C, looking down), the last president of U.S.-back Saigon regime, and his entourage leave the presidential palace in Saigon April 30, 1975 after announcing on radio his cabinet's surrender following his capture by North Vietnamese communist troops.

Jubilant communist troops on top of trucks and APCs making their way to the center of Saigon as the city fell under their control, April 30, 1975.

PRG  forces seize control of the presidential palace in Saigon, after the fall of the city, May 3, 1975.

The Viet Cong created the Provisional Revolutionary Government of the Republic of South Vietnam (PRG) in 1969.

The war in Vietnam ended April 30, 1975 as the government in Saigon announced its unconditional surrender to the Vietcong.

Residents look at U.S. army guns collected in a Saigon street, after the fall of the city, May 27, 1975.

The war in Vietnam ended April 30, 1975 as the government in Saigon announced its unconditional surrender to the Vietcong.

A National Liberation Front Army soldier (NLF) (L) patrols the streets of Saigon, after the fall of the city, 17 May 1975.

That NLF, formally known as the National Front for the Liberation of the South, was created by North Vietnam December 20, 1960 to effect the overthrow of the South Vietnamese government and reunification of North and South Vietnam. The Vietnamese communist party was later established in 1962 as a component of the NLF. The Viet Cong, the military arm, was part of the NLF.

More photos: U.S. Withdraws from Vietnam