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U.S. airmen enclosed the besieged fortress in a virtual curtain of falling bombs. Though the Marines lost most of their original supply of artillery ammunition when an enemy shell hit their supply dump early in the siege, they were able to call in airpower for the sort of pinpoint destruction that is normally associated with howitzers. When the lowering clouds lifted a few hundred feet, dartlike Air Force F-100s, Navy and Marine F-4 fighter-bombers and stubby A-4 light bombers zipped under the overcast to place high explosives on the spreading enemy trenches. Huge, eight-jet B-52s, which bomb by radar, flew over Khe Sanh regardless of the weather.
At first, Khe Sanh's barren landscape presented problems for the B-52s' radar system, which usually takes a fix on a prominent ground feature, such as a bridge or high building. To solve that, the U.S. employed a recently developed system called "Sky Spot." Using a power ground-control center on South Viet Nam's coast, Sky Spot directed the bombers to the general area of their destination. There, on hilltops miles from the fighting, the U.S. placed meshes of wire that acted as radar reflectors and electronic beacons that emitted continuous signals. Gauging the distance to their targets from these spots, the B-52s were able to bomb with uncanny accuracy; the big bombers, in fact, were able to walk their sticks of bombs to within 100 yds. of the perimeter of the Marine bastion. Flying the 5,200 mile round trip from their Guam base, they averaged 40 to 50 strikes each day. Hardly an hour passed without a bombload falling on the Communists. In ten weeks, a total of 103,500 tons of explosives were dumped on the five-mile-square battlefield around the base.
The bombing struck dread into the North Vietnamese. They feared the fighter-bombers, but most of all they feared the B-52s. Reason: the B-52s fly so high—above 40,000 ft.—that their approach is unknown to those on the ground until the huge bombs fall on them. According to the U.S. estimates, 15,000 enemy troops were killed or injured by U.S. bombardment. The bombs obliterated trenches, leveled hills, scorched whole acres of land. They even wiped out the North Vietnamese headquarters bunker, killing all those inside. The bombing touched off 5,000 secondary explosions and more than 2,000 fires in the immediate vicinity of Khe Sanh, indicating that ammunition and gasoline caches were being hit hard. In all, the Air Force estimates that the bombing destroyed 3,500 tons of Giap's supplies—enough to sustain a full division in combat for a month.
Some time around March 12, the day before the 14th anniversary of his victory at Dienbienphu, General Giap seems to have come to the conclusion that he would not be able to repeat his earlier feat, and he stopped sending replacements to Khe Sanh. Then, on March 22, he ordered one of his two battered divisions around Khe Sanh to withdraw.