President Johnson ended the bombing of North Vietnam in October 1968. believing that USAF unarmed reconnaissance aircraft would be permitted to fly over the country unopposed. When North Vietnam fired on some of these planes, newly elected President Richard Nixon ordered several retaliatory strikes against air defense sites.
Still, North Vietnam imported materiel by rail from China or by sea through North Vietnam's ocean ports (primarily Haiphong) without fear of attack. Furthermore, the rainy season that began in May greatly limited the effectiveness of U.S. air attacks, and North Vietnam began stockpiling supplies and equipment inside its border with Laos -- it even built an oil pipeline into the panhandle of Laos.
Although the bombing halt continued throughout 1970, the USAF attacked antiaircraft and SAM sites that fired at unarmed reconnaissance aircraft. Because of repeated SAM firings, Washington directed the USAF to intensify its retaliatory strikes. From February through September 1971, USAF fighter-bombers attacked SAM sites, enemy road construction through the DMZ, and oil storage facilities. Most of these missions were in southern North Vietnam, leaving Hanoi to continue its build-up in the north. By late in the year, the North Vietnamese Air Force's MiGs had become a significant threat. As a result, Washington authorized the USAF and Navy to bomb the three MiG airfields in southern North Vietnam, and in November, the U.S. "neutralized" these bases.
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