WASHINGTON — President Nixon, in his first year in office and eager to end an unpopular war that killed tens of thousands of U.S. troops, considered using nuclear weapons against the North Vietnamese, recently declassified documents show.

By mid-1969, Nixon and national security adviser Henry Kissinger had settled on a strategy using international diplomacy with threats of force against the communists ruling the north in an attempt to get them to buckle, according to an analysis of the papers by the National Security Archive. The private research group is headquartered at George Washington University.

Kissinger and his staff began developing contingency military plans under the code name of "Duck Hook." He also created a committee within the National Security Council to evaluate secret plans prepared by Joint Chiefs of Staff in Washington and military planners in Saigon.

A pair of declassified documents raised the question of nuclear weapons use in connection with the military operation against the north, which was fighting to reunite with the democratic south, according to the archive.

The first is a Sept. 29, 1969, memo from two Kissinger aides — Roger Morris and Anthony Lake — to Capt. Rembrandt Robinson, who had a central role in preparing the Duck Hook plans. Robinson had prepared a paper for the NSC committee outlining the Joint Chiefs plans to attack North Vietnam.

But the archive says Morris and Lake, unhappy with the document, asked Robinson to rework it to present "clearly and fully all the implications of the (Duck Hook) action, should the president decide to do it."

They said the president needed to decide in advance "the fateful question of how far we will go. He cannot, for example, confront the issue of using tactical nuclear weapons in the midst of the exercise. He must be prepared to play out whatever string necessary in this case."

The second document is an Oct. 2, 1969, memo from Kissinger to Nixon, introducing an NSC staff report on the state of military planning for Duck Hook. The report said the basic objective of the operation would be to coerce Hanoi "to negotiate a compromise settlement through a series of military blows," which would walk the fine line between inflicting "unacceptable damage to their society" and causing the "total destruction of the country or the regime."

But Nixon abandoned Duck Hook shortly after Oct. 2. Both his secretaries of Defense and State, Melvin Laird and William Rogers, opposed the plan. Nixon apparently also began to doubt whether he could sustain public support for the three- to six-month period the plan might require. He also concluded that his military threats against the North Vietnamese had no effect.

U.S. troops remained in the country throughout Nixon's first term despite a gradual withdrawal of forces that he began in 1969. Nixon was re-elected in 1972 and secured a cease-fire agreement the following year, but it was never implemented.

Two years later, in 1975, North Vietnamese forces overran the South, reuniting the country under Communist rule.