WASHINGTON (AP) _ The United States had explicit plans for dropping the atomic bomb on China in 1954 if the Chinese violated the tenuous truce that ended the Korean War, according to a newly declassified Pentagon document.

The April 17, 1954, memo showed the extent to which the Eisenhower administration was ready to use nuclear weapons in enforcing Secretary of State John Foster Dulles' Cold War policy of ''massive retaliation.'' The document was signed by Brig. Gen. Edwin H.J. Carns, secretary to the Joint Chiefs of Staff.

''In light of the enemy capability to launch a massive ground offensive, U.S. air support operations, including use of atomic weapons, will be employed to inflict maximum destruction of enemy forces,'' the memo said, detailing the U.S. response for the war's resumption with Chinese forces again massively involved.

The document also showed that the United States planned to blockade China's coasts, seize offshore islands and use Chinese Nationalist forces to stage raids on the mainland in the event of renewed hostilities.

The memo - of which only 30 copies were made, each numbered - was among 44 million documents from World War II and the postwar years and from the Korean and Vietnam wars that were declassified in a blanket order signed by President Clinton last month.

They were opened to the public Monday at the National Archives. Together they represent 14 percent of the remaining secret papers from those eras - one of the biggest mass declassifications in U.S. history.

Dwight Eisenhower was elected in 1952 on an implied pledge to end the long and bloody Korean conflict and to keep American soldiers out of future land wars in Asia. The United States had lost 33,629 lives in the three years of Korean fighting and the American public had no appetite for further bloodshed.

Historians have known for some time that the Pentagon was planning for the possible use of nuclear weapons, especially tactical nuclear weapons, if the war resumed with large-scale Chinese participation.

''This document contributes to the ongoing historical debate over how serious the consideration of nuclear use was during this period,'' said Jim Hershberg, director of the Cold War International History Project at the Woodrow Wilson Center here.

''There is still controversy over what role the Eisenhower administration's nuclear threat played in bringing the war to an end,'' he said. Some historians believe Eisenhower had decided to go nuclear if the war resumed; others say he was merely prudently keeping his options open.

In their memoirs, Eisenhower and Dulles claimed ''that their use of nuclear brinksmanship - that is their threat, secretly and indirectly conveyed, to use nuclear weapons against China if the war resumed - was crucial in hastening the speedy end of the negotiations, which had dragged on for almost two years,'' Hershberg said.

''However, recently discovered documents from Soviet archives suggest that (Soviet dictator Josef) Stalin's death in March 1953 may have been more important in causing the Soviets and Chinese to agree on the need to rapidly conclude the conflict.''

The atomic bomb has been dropped only twice, against the Japanese cities of Hiroshima and Nagasaki, killing a total of 210,000 people and leading to a swift end of World War II.

The Pentagon memo appeared to reflect worldwide sensitivities about the use of the bomb on civilian populations, stressing that under the plans, the use of atomic weapons ''will be strictly limited and at no time is a mass atomic bombing envisaged.''