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.