THE rhythm of summitry between US and Soviet leaders provides a remarkably accurate reflection of the shifting moods of relations between the two nuclear-armed superpowers.When tensions rose, summits were few and far between. The high-level get-togethers picked up in the era of detente. Until recently, their talk focused on the weapons of war, on creating and preserving a precarious balance of terror. After World War II, US leaders had to wait until two years after the death of Soviet dictator Josef Stalin to meet Soviet leaders. Leaders of the "Big Four" (the US, Soviet Union, France, and Britain) met in Geneva in 1955. Emerging from Stalinist isolation, the Soviet collective leadership of Nikita Khrushchev, Nikolai Bulganin, and Viacheslav Molotov offered a new flexibility, labeled the "spirit of Geneva," and proposed talks on European security and limiting nuclear weapons. A bilateral summit did not occur until Khrushchev's 1959 trip to the US, typified by his visit to an Iowa corn farmer. The good feelings rapidly disappeared when Moscow canceled a May 1960 meeting in Paris, after an American U-2 spy plane was downed and its pilot captured. The meeting with newly elected President John F. Kennedy in Vienna in June ended almost as badly, with angry exchanges. "Khrushchev felt communism was on the rise," explains Soviet historian Vladislav Zubok, a cold war specialist. US and Soviet leaders did not meet again until Lyndon Johnson met Premier Alexei Kosygin in Glassboro, New Jersey, in June 1967. (See story, right.) A summit in Leningrad in December 1968, to begin talks on the first treaty to limit nuclear arms (SALT), was scrapped after the Soviets invaded Czechoslovakia in August. Relations did not revive until Richard Nixon became the first US President to visit the Soviet Union in May 1972. Leonid Brezhnev sacked Ukrainian party leader Pyotr Shelest only days before Nixon's arrival for opposing the visit of "the butcher of Vietnam," recalls Mr. Zubok. The signing of SALT I - the Anti-Ballistic Missile (ABM) treaty - and a bilateral trade agreement at this summit was a high point of detente. Brezhnev returned the visit in 1973, agreeing to try to complete a SALT II treaty by the next year. Nixon's subsequent visit to Moscow in 1974, a month before his forced resignation, yielded little. In November 1974, Gerald Ford and Brezhnev agreed on the framework for SALT II at a brief meeting in the Soviet Pacific port of Vladivostok. President Jimmy Carter and Brezhnev signed the treaty in Vienna in June 1979, but it was never ratified by the US Senate. The Soviet invasion of Afghanistan in December ushered in a new freeze in relations. After four years in office, Ronald Reagan met Mikhail Gorbachev in Geneva in November 1985, and they agreed to resume nuclear-arms negotiations. A hasty summit in Reykjavik, Iceland, in October 1986 ended when Reagan refused to end the Star Wars anti-missile program. Mr. Gorbachev visited Washington in December 1987 to sign the treaty to eliminate intermediate-range nuclear forces (INF). Reagan visited him in May and June the next year. "They returned to the status quo," says Zubok, reinstating agreements frozen after the Afghan invasion. The Bush era has seen three summits. The first shipboard summit in Malta in December 1989 was a cautious affair but led to acceleration of talks on limiting conventional forces in Europe and conclusion of the Strategic Arms Limitation Treaty. The Gorbachev visit to Washington the next year focused on German reunification and nuclear disarmament. The first truly post-cold-war summit may have been the one-day meeting in Helsinki in September to discuss coordinated policy on the Gulf war. The two men met again at the Paris conference on European security in November and earlier this month at the London Group of Seven summit.