The biggest ever bombing campaign by US B-52 bombers took place over Christmas 40 years ago, when the US dropped 20,000 tonnes of explosives on North Vietnam. More than 1,000 Vietnamese died, but the assault may have helped bring about the peace deal signed a month later.
Operation Linebacker II was President Richard Nixon's attempt to hasten the end of the Vietnam War, as the growing strength of the Viet Cong caused heavy casualties among US ground troops.
The capture and torture of downed airmen in the north, regularly paraded on television, was also an embarrassment for Washington. Nixon was under pressure to bring the troops home.
At the same time, long-running negotiations in Paris between the warring parties had broken down.
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