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 the 6th and the 15th, 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 the b-52s were then withdrawn from operations in the north, and when they returned in june, their missions would be limited to south vietnam.