U.S. Secretary of Defense Robert S. McNamara announces plans to build an electronic anti-infiltration barrier to block communist flow of arms and troops into South Vietnam from the north at the eastern end of the Demilitarized Zone (DMZ). The “McNamra Line,” as it became known, would employ state-of-the-art, high-tech listening devices to alert U.S. forces when North Vietnamese troops and supplies were moving south so that air and artillery strikes could be brought to bear on them. It was estimated that the cost of completing and maintaining the project would be more than $800 million per year. Construction on the barrier line, initially code named “Practice Nine” and later changed to “Dye Marker,” began almost at once. But in the end, the concept proved impractical as the North Vietnamese just shifted their infiltration routes to other areas.