an average of 22,000 tons of supplies a month over two rail lines and eight major roads that linked it with North Vietnam. On 10 May Operation Linebacker began with mass bombing operations against North Vietnam by tactical fighter aircraft of the Seventh Air Force and Task Force 77. Their targets included the railroad switching yards at Yên Viên and the Paul Doumer Bridge, on the northern outskirts of Hanoi. A total of 414 sorties were flown on the first day of the operation, 120 by the Air Force and 294 by the Navy and they encountered the heaviest single day of air-to-air combat during the Vietnam War, with 11 VPAF MiGs (four MiG-21s and seven MiG-17s) and two Air Force F-4s shot down. Anti-aircraft artillery and over 100 surface-to-air missile firings also brought down two USN aircraft (one of which was flown by aces Duke Cunningham and William P. Driscoll). By the end of the month, American aircraft had destroyed 13 bridges along the rail lines running from Hanoi to the Chinese border. Another four were destroyed between the capital and Haiphong, including the notorious Thanh Hóa Bridge. Several more bridges were brought down along the rail line leading to the south toward the DMZ. Targets were then switched to petroleum and oil storage and transportation networks and North Vietnamese airfields. There was an immediate impact on the battlefield in South Vietnam. Shelling by PAVN artillery dropped off by one-half between 9 May and 1 June. This slowdown was not due to an immediate shortage of artillery shells, but rather to a desire to conserve ammunition. U.S. intelligence analysts believed that PAVN had enough stockpiled supplies to sustain their campaigns throughout the autumn. The intensity of the bombing campaign was reflected by the sharp increase in the number of strike and support sorties flown in Southeast Asia as a whole: from 4,237 for all services, including the RVNAF, during the month preceding the invasion, to 27,745 flown in support of ARVN forces from the beginning of April to the end of June (20,506 of them flown by the Air Force). B-52s provided an additional 1,000 sorties during the same period. The North was feeling the pressure, admitting in the official PAVN history that "between May and June only 30 percent of supplies called for in our plan actually reached the front-line units." In total, 41,653 Linebacker missions dropped 155,548