North Vietnamese forces block Routes 1, 4, and 13, all major South Vietnamese ground supply routes to Saigon. For the next two months, Communist forces repeatedly interdicted these and other key supply routes critical to Saigon's survival in an attempt to strangle the city. This was all part of the Nguyen Hue Offensive, which had been launched in late March.
In an invasion by more than 120,000 communist troops, the North Vietnamese had taken Quang Tri and lay siege to An Loc and Kontum. Despite desperate fighting on a level heretofore unseen in the war, the South Vietnamese forces, with American advisors and U.S. tactical air support, had withstood the invasion and were preparing to retake Quang Tri. At one point, the North Vietnamese forces had been less than 60 miles from Saigon, but were stopped by the South Vietnamese forces at An Loc, on Highway 13 north of the city.