IN the early days of April 1975, then-Maj. Gen. Nguyen Cong Trang of the North Vietnamese Army was worried about bridges.
Because of a successful diversionary tactic at the city of Buon Me Thuot in what was then known as South Vietnam, Northern forces had taken the key central Vietnamese cities of Hue and Da Nang in the last days of March with relative ease.
Now Major General Trang had to get his corps down to Saigon, now known as Ho Chi Minh City, to be in place for the final campaign of the war. That meant moving thousands of troops some 600 miles in 27 days.
But the retreating Southern troops were destroying bridges as they fled and there are a lot of rivers in Vietnam.
''To reach Saigon in such time,'' Trang recalled in an interview at a veterans' center in Hanoi, ''we had to move the troops by vehicles -- trucks and cars.... We ordered our troops that [those in] each car must cook their own food. The advance must be nonstop.''
Engineering brigades carted sandbags from abandoned South Vietnamese Army posts to serve as pilings for bridges or tied boats together as pontoons.
The North Vietnamese troops, racing to take the Southern capital before May on orders from their leaders in Hanoi, fought several engagements on their hurried trip south.
Ever mindful of the possibility that American forces might return to protect the Southern government, the Northerners took the Saigon airport first. ''If the airport is controlled by us,'' Trang says, ''they cannot return.''
From the airport, tanks and troops sped to the Presidential Palace in Saigon at dawn on April 30 to receive the surrender of the Saigon government.
By 11:15 a.m., Gen. Duong Van Minh, recently installed as president of South Vietnam, was reading the surrender on national radio.
Trang, now retired but wearing an old uniform shorn of its insignia, cites three reasons for the North's victory:
* Vietnamese nationalism.
* The mobilization of civilians in both North and South.
* ''The support and assistance of people the world over,'' including, he says, the American antiwar movement.