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New technologies proved crucial in America’s fight against the Viet Cong.

The Vietnam War saw a unique combination of guerrilla and conventional warfare, both of which depended on timely intelligence for success. The United States used sophisticated and constantly evolving electronics to intercept radio communications and spot enemy positions. While North Vietnam protected its skies from American fighter jets and bombers by deploying Soviet-supplied radar, weapons and equipment, Americans pushed the envelope with airborne radar aboard specially modified aircraft. On land, America’s AN/TPS-25 ground surveillance radar and AN/MPQ-4 counterbattery radar detected troop movements and artillery.

In a $1.5 billion program to stem the flow of reinforcements and materiel from the North, the U.S. started—but never finished—a barrier of  old-fashioned barbed wire and mines, plus electronic sensors, near the Demilitarized Zone. It was called McNamara’s Line, after Secretary of Defense Robert McNamara. Electronic sensors and odor-detection methods to identify human targets were only partially successful. For example, the Viet Cong foiled devices that detected the ammonia in sweat by hanging urine bags from trees so the Americans would bomb those areas while they conducted their business elsewhere. Both sides also had traditional human spies and intelligence collectors, but the Viet Cong had the advantage of spies who could easily blend in with people friendly to Americans.