United States Air Force B-52 bombing crews at Andersen Air Base in Guam being briefed on the U.S.’s final major aerial bombing campaign in North Vietnam, Operation Linebacker II.  – Wikimedia Commons

On December 13, North Vietnamese negotiators walked out of secret talks with National Security Advisor Henry Kissinger. President Richard Nixon issued an ultimatum to Hanoi to send its representatives back to the conference table within 72 hours “or else.” The North Vietnamese rejected Nixon’s demand and the president ordered Operation Linebacker II, a full-scale air campaign against the Hanoi area. American antiwar activists dubbed Linebacker II the “Christmas bombing.” – History.com