air bases around the globe, American forces were put on alert and ships and aircraft squadrons began moving toward Southeast Asia. The U.S. began a rapid build-up of airpower. The USAF deployed 176 F-4s and 12 F-105 Thunderchiefs from bases in the Republic of Korea and the U.S. to Thailand between 1 April and 11 May in Operation Constant Guard. Strategic Air Command (SAC) dispatched 124 B-52s from the U.S. to Guam bringing the total B-52 strength available for operations to 209. The USN cut short its port period for the carriers and and ordered , and to augment the fleet so that four or more carrier air wings could conduct missions simultaneously. 7th Fleet assets in local waters were thereby increased from 84 to 138 ships. USAF tactical strikes against North Vietnam north of the 20th parallel were authorized on 5 April under the nickname Freedom Train. The first mass B-52 raid directed against the north was conducted on 10 April when 12 B-52s, supported by 53 attack aircraft, struck petroleum storage facilities around Vinh. By 12 April, Nixon had informed Kissinger that he had decided on a more comprehensive bombing campaign which would include strikes against both Hanoi and Haiphong. The following day 18 B-52s struck Thanh Hóa's Bai Thuong Air Base. Three more days followed before another strike, this time by another 18 bombers in a pre-dawn attack against an oil tank farm outside Haiphong. They were followed by more than 100 tactical aircraft attacking targets around Hanoi and Haiphong during daylight. Between 6 and 15 April, U.S. aircraft also struck and destroyed the Paul Doumer and Thanh Hóa bridges and the Yên Viên railway marshalling yard. This marked the introduction of laser-guided bombs against strategic targets in North Vietnam. Both bridges had previously been attacked unsuccessfully with conventional bombs and even missiles. The B-52s were then withdrawn from operations in the north and when they returned in June, their missions would be limited to South Vietnam. By mid-month, nearly all of North Vietnam had been cleared for bombing raids for the first time in over three years. Air Force and Navy commanders and pilots were relieved that Nixon (unlike President Johnson) left the operational planning to local commanders and loosened the targeting restrictions that had hampered Operation Rolling Thunder. Between 1 May and 30 June B-52s, fighter-bombers, and gunships had flown 18,000 sorties against formidable anti-aircraft