For President Richard M. Nixon, flying into Tehran from Moscow on May 30, 1972, was welcome relief, "Getting out of the bush leagues" was how Dr. Henry Kissinger, his national security adviser, put it. They had just spent eight days in what Nixon called "absolute purgatory" in the Kremlin, painstakingly negotiating the SALT I treaty with the Soviet Union limiting strategic weapons of mass destruction. It had been a wearing but finally triumphant spring, dominated by the North Vietnamese Easter offensive against South Vietnam, which had been stopped only by massive U.S. bombing and the mining of North Vietnam's ports. So controversial were Nixon's decisions about Vietnam—and so apparently humiliating for the Soviet Union, North Vietnam's principal backer—that the Moscow trip had hung in the balance until the last moment.