In May, 1972, U. S. forces mined North Vietnamese ports following large scale employment of North Vietnamese Army units in South Vietnam. This action completely shut down seaborne importations to North Vietnam. The logistics train needed to support these Army units was crippled by elimination of 85% of imported supplies for the North Vietnam war effort. This forced war material imports to enter the country and move south by train, during daylight hours, in addition to night hours, making them much more susceptible to successful air attack. The combination of mines and B-52 bombing significantly reduced the capabilities of North Vietnam Army units in South Vietnam. Reseeding of mine fields and resumption of B-52 bombing in December 1972 followed further intransigence by North Vietnam in reaching a cease-fire agreement.

 Eventually, a cease-fire agreement was reached in January, 1973, contingent on U. S. forces’ clearance of mines. Clearance began in February, 1973. Further balking by North Vietnam, in complying with the cease fire agreement and release of American POW’s, was answered by interrupting mine clearance, resulting in release of POW’s by 1 April 1973.  Combined with diplomatic isolation of North Vietnam from the USSR and the PRC and B-52 bombing, mining was clearly a key ingredient in dealing effectively with a previously intransigent government.