Working in a tiny underground bunker, the handful of intercept operators pinpointed enemy infiltrators, artillery units moving toward the border, and mobile surface-to-air missiles through voice and coded intercepts. "In A-4 we were in a bunker underground," said the intercept operator. "They had the codes broken, they could pick up the firing designators. When the North Vietnamese got on the radio to open up the guns or the rocket attack, they would use designators. And the Americans knew the designators, so we would know when we were about to get shelled and we would go back underground so we didn't get blown up."

The concrete bunker was about ten feet underground and held only about five to seven intercept operators. Five worked the intercept equipment while the other two slept. They would take turns and they were all volunteers. Nearby was another bunker containing the NSA Explorer remote intercept equipment.

In early 1972, the intercept operators at A-4 began getting indications of something larger than the usual infiltration or harassment taking place across the border. "We thought there was going to be an invasion, and nobody was really listening," said one intercept operator that was there at the time. "That was January, February, beginning of March, 1972. There was just too much build up of activity above the DMZ for it not to happen. We were reporting that to the higher-ups. But in my personal opinion, it fell on deaf ears because at that time there weren't any Americans except for the intelligence people and then the few American advisors who were up there."

Further to the west, at Firebase Sarge, indications of a major attack were also becoming more numerous. There, the only Sigint personnel were two Army specialists, Bruce Crosby, Jr., and Gary Westcott, assigned to maintain the Explorer equipment contained in a bunker. The only other American was Marine Major Walter Boomer, who was an advisor to South Vietnamese forces assigned to the firebase. Earlier in March, Boomer had warned General Giai, the commanding general of the South Vietnamese Army's 3rd Division, of his deep concern about the steady increase in enemy activity in the area. He told Giai that he felt that something significant was going to take place soon. The general listened but said there was little he could do.

To the south, at Cam Lo, a secret American facility monitored the DMZ through ground surveillance devices planted throughout the zone. During most of March, the number of trucks detected crossing the DMZ had tripled, and the monitors recorded both wheeled and tracked vehicle traffic, a worrisome sign. By the end of the month, the monitors were recording heavy traffic even during daylight hours, something that had never happened before.

The bad news came on Good Friday, March 30, 1972. Just before noon on Firebase Sarge, Major Boomer passed on to his headquarters some disturbing news. "Shortly after daylight the NVA began to shell us here at Sarge." he said. "The NVA's fire is as accurate and as heavy as we have ever experienced up here. We're all okay now, but there is probably a big battle coming our way....It looks like this could be their big push."

It was TET all over again. The North Vietnamese Army had launched their largest offensive in four years, and US and South Vietnamese forces were just as unprepared as they had been the last time. In fact, the U.S. military command in Saigon, 350 miles south, refused to believe a major attack was in progress even after it had begun. Over 30,000 well-armed soldiers supported by more than 400 armored fighting vehicles, tanks, mobile missle launchers, and long-range cannons poured over the DMZ. Crossing the Ben Hai River, they knifed into the South's Quang Tri Province and turned the lonely firebases, like islands in the sky, into shooting galleries.

Up on Firebase Sarge, as the earth rolled from the violent assault, Boomer ordered Westcott and Crosby to remain in the NSA Explorer bunker and keep in radio contact with him and also with the listening post at A-4. Explorer was housed in an aluminum hut that also contained 8 pieces of NSA crypto equipment. Around the hut was a bunker made of several rows of sandbags and a steel roof covered with another 5 feet of additional sandbags. For ventilation, there was a window on one side.

Below Sarge, Soviet 130mm guns, the size of telephone poles, let loose with boulder-sized shells. The rattle of small-arms fire followed and then the heavy crump of 122mm rockets raining down. Suddenly both A-4 and Boomer lost contact with Westcott and Crosby. Shortly after noon, a rocket scored a direct hit, crashing through the window in the NSA Explorer bunker. The two intercept operators were killed instantly and the bunker became a crematorium, burning for days. More than a decade after the first Sigint soldier died in Vietnam, two of the last were killed.

With A-4 also under heavy assault, the intercept operators were ordered to begin destroying Explorer and the rest of the crypto equipment and files. Above each of the sensitive devices were thermite plates for quick destruction. The plates were electrically activated and were wired together to a switch on the outside of the hut. Each thermite plate--about a foot wide and an inch thick--was designed to burn at the solar-like temperature of 35,000 degress Fahrenheit. "The hut would burn for a couple days before all the metal essentially turned to ash," said one of the soldiers who installed the destruction devices. "Once the thermites reached full temperature and the hut started burning no one could possibly survive and in the end there would be nothing left, absolutely nothing." Within a day of what became known as the Easter Offensive, there was no evidence that NSA had ever been at A-4, just ashes. The war was over and the United States had lost.

On January 27, 1973, the United States and Vietnam signed a cease-fire agreement at 7:45 A.M., fifteen minutes before the cease-fire took effect, the USS Turner Joy, which had helped launch America's misguided adventure, sailed off the Cam-Lo-Cau Viet River outlet and senselessly fired off the last salvo of the war.