 Complete the following phrase:  In North Vietnam, American bombing of railways was concentrated on key targets such as bridges, both along the NorthSouth Railway and along the lines north of Hanoi, such as the HanoiLào Cai and HanoiDong Dang lines. At several points during the war, American bombing severely hindered transport along both lines, on which the North Vietnamese depended for shipments from their Chinese allies. Operation Rolling Thunder was the first large-scale bombing campaign carried out by the U.S. Air Force, taking place from March 2, 1965 until November 1, 1968, when US President Lyndon B. Johnson temporarily called off air raids. Large-scale air raids resumed from May 9 to October 23, 1972, for Operation Linebacker, and again from December 1829, 1972, for 