Worried that then-South Korean President Syngman Rhee might undermine negotiations to end the Korean War, the United States gave serious consideration to implementing top secret plans to overthrow its "often unreliable ally," according to declassified Korean War-era documents.

The plan, called Operation Ever-ready, was never put into effect, the documents show. It was one of three options considered by top-level U.S. officials in mid-1953 as possible means of overcoming Rhee's objections to portions of the proposed armistice agreement.

The other two options were negotiating the bilateral security pact with South Korea that Rhee strongly favored or pulling U.S. troops out.

The option chosen was to negotiate the security pact that remains the basis for the current presence of U.S. troops in Korea. President Carter has proposed a five-year staged withdrawal of those troops.

Documents relating to Operation Everready were made available to The Washington Post by Barton Bernstein, a Stanford University history professor: Others were obtained independently.

According to Bernstein, "the reason they didn't use it [Everready] is because there was no alternative to Rhee no good, strong anti-Communist leader to replace Rhee with."

Gen. Mark W. Clark, who was commander in both the U.S. and United Nations troops in Korea, was in structed by the joint chiefs of staff in mid-1952 to develop a contingency response to Rhee's threats to pull South Korea's troops from the U.N. forces. South Koreans represented about 60 per cent of U.N. troops at the time, Bernstein said.

The plan Clark devised on May 4, 1953, anticipated hostile actions by South Korean troops toward U.N. forces. It included among its possible measures: "Proclaim and establish martial law in the name of U.N.: secure custody of the dissident civilian and military leaders: proclaim military government in the name of the U.N."

The plan is spelled out in greater detail in an undated memo from Clark to the Department of the Army. It states. "I am prepared to take action in case widespread disorders affecting my mission break out. . .

"President Rhee would be invited to visit Seoul or elsewhere - anywhere to get him out of Pusan," the temporary capital.

"At an appropriate time, the U.N. commander would move in to the Pusan area and seize between five and ten key ROK (Republic of Korea) officials who have been leaders in Rhee's dictatorial actions . . . and take over control of martial law through the Chief of Staff ROK Army until it is lifted.

"Rhee would then be informed of the action taken as a 'fait accompli.' He would be urged to sign a proclamation lifting martial law, permitting National Assembly freedom of action, and to establish freedom of the press and radio without interference from his various strong armed agencies.

"If President Rhee would not agree to issue the proclamation he would be held in protective custody, incommunicado, and a similar proclamation would be presented to Prime Minister Chang Tack-Sang."

Rhee was opposed to ending the war, it is generally known, and wanted U.N. troops to push the fighting north.

In mid-1953, the time of the most serious consideration of putting the plan into effect. Rhee was adamantly opposing North Korean demands for forced repatriation of the 140,000 North Korean and Chinese prisoners of war in South Korea.

Eventually, about 20,000 of the POWs were allowed to remain in South Korea or in Taiwan.

At his home in South Carolina yesterday, Clark said Operation Everready was given serious consideration, but "it would've been a tough job to do. Old man Rhee was a tough character . . . His people would have supported him."

Rhee was overthrown by protests and student riots in 1960s. He died in 1965 in Honolulu at the age of 95.

Bernstein said he is familiar with no case that resembles that of Rhee except for the overthrow of South Viet Nam's Premier Ngo Dinh Diem in 1963.