The NVA was attempting to repeat their famous victory at Dien Bien Phu, but the overwhelming power of US air support and vastly increased airlift capacity made their attempt to starve out the base a failure. The NVA eventually adbandoned their assaults at the start of the Tet Offensive after heavy casualties on both sides.
The base had its origins in September 1962, when an airstrip was constructed outside the town of Khe Sanh, about 7 miles from the Laos border. The airfield saw little use until a Special Forces team constructed a base next to the airfield in 1965, a base that would become the Khe Sanh Combat Base, scene of the battle. Over the next few years the base was used as a staging ground for a number of attacks on troop movements down the Ho Chi Minh trail, and was permanently manned by Marines starting in 1967. A smaller Special Forces base was later constructed down the road toward the Laos border, known as Lang Vei, and was in the process of being moved about a kilometer further west when the battle began.
In 1968 General William Westmoreland decided that Khe Sahn would be the site to use in an attempt to bring the NVA into direct confrontation. By sending in a massive re-enforcement force he planned on launching major operations against the Ho Chi Mihn trail, which would effectively cut off all NVA operations further south, he planned on forcing them to attack Khe Sahn in order to re-open the trail. If his plan was successful, he believed, that the war would be soon over. However, the offensive potential of Khe Sanh never materialized.
During the American build-up, North Vietnamese forces were finding excellent defensive positions on nearby hills that were heavily fortified with caves and former mines that were practically impervious to attack. Over a period of just over a week three full divisions of about 25,000 men were moved into the area, well supported from the nearby Trail. From these positions they launched mortar and rocket attacks on the base, "covered" to a great degree by continuing bad weather.
The main assaults on the base started on February 5th, 1968. On the 7th a major tank-supported assault overran Lang Vei in spite of the heroic defence of what was essentially an undefendable position. A number of massive attacks on Khe Sanh took place over the week, but eventually it became clear the Marines positions were well developed. The buildup nevertheless continued on both sides, and while the US troops were eventually ready to launch small offensives against the NVA forces pouring into the area, they were unable to do so due to the heavily forested areas in the valleys between themselves and the fortified hills.
After this the tempo slowed and the battle became more of a siege, with the almost continual artillery duels soon turning the base into a huge trench system looking more out of World War I than Viet Nam. The US turned to air power as a way out of the stalemate, and called in huge bombardments on the hills by B-52's flown from Okinawa. Soldiers on both sides still express awe to this day when talking about them; the attacks gave absolutely no warning, and suddenly an entire hill would be completely covered with exploding bombs. Meanwhile losses to artillery falling on the base were being made up by new supplies being flown in, one soldier noted that a fresh batch of light 105mm howitzers would be wiped out by NVA counterfire by the end of a day, only to be replaced by the next morning when it would start all over again. Attempts by the NVA to shut the runway were never entirely successful, and somewhat superfluous due to the massive number of helicopters the US had at their disposal even if this were to occur.
Efforts on the part of the NVA to reopen the battle soon started following their earlier successful efforts at Dien Ben Phu, and they started the construction of a major trenchwork/tunnel system in an attempt to enter the base under cover. However the airpower available to the US and French forces a decade earlier were of an entirely different nature, whenever a trench system came anywhere near the base a B-52 strike would turn the entire area into a moonscape, completely erasing the engineering efforts.
Two further major assaults followed on March 17-18th and the 29th. Both were repulsed, the second one with ease, and it was now clear that the base would not fall due to any actions the NVA could bring forth. At this point the NVA divisions were recalled and the battle slowly ended. The Americans held Khe Sanh throughout the siege, and were eventually officially relieved by the 2nd Cavalry on April 6th, 1968, and all fighting was over two days later.
As a military action Khe Sanh was another costly failure on the part of the NVA, with estimates of 8,000 NVA dead and considerably more wounded, likely the overwhelming majority of the forces sent to the area. This must have been particularly galling given the similarities to Dien Ben Phu, there is no doubt that they did seriously intend to win the battle.
Nevertheless the battle was a major strategic win. As the leadup to the battle took place over late 1967 and into January 1968, the entire American military system swung into a singular effort to "win" the battle. Although plenty of intelligence suggested that a large scale effort was being planned all over Vietnam, this information was ignored. This too was a part of the Khe Sanh plan, one that was executed perfectly, leaving the Americans completely surprised by Tet.
In the end the battle decided nothing, but proved to both sides that the war they were involved in required new tactics. Khe Sanh itself was abandoned on June 23, 1968 since it no longer had any military value.