 Because of Thieu's unhappiness with the agreement, primarily the stipulation that North Vietnamese troops could remain "in place" on South Vietnamese soil, the negotiations in Paris stalled as Hanoi refused to accept Thieu's changes and retaliated with amendments of its own.To reassure Thieu of American resolve, Nixon ordered a massive bombing campaign against North Vietnam utilizing B-52s and tactical aircraft in Operation Linebacker II, which began on December 18 with large raids against both Hanoi and the port of Haiphong.Nixon justified his actions by blaming the impasse in negotiations on the North Vietnamese.