On March 30, 1972 some 40,000 North Vietnamese troops along with tanks and heavy artillery surged across the demilitarized zone into South Vietnam in the opening round of Hanoi's Easter Offensive. My early May, without the support of U.S. ground combat troops, South Vietnamese forces were on the ropes. In response, President Richard Nixon called for an aggressive, sustained bombardment of North Vietnam. Codenamed Operation Linebacker, sought to the interdiction effort sought to stem the flow of men and materiel southward, as well as sever all outside supply lines in the first bombing of the North Vietnamese heartland in nearly four years.