Over the forty-two years of official relations between the United States and the Republic of India, periods of relative goodwill and mutual understanding have alternated with phases of disappointment and irritation on the part of one or the other of the two parties—and, often, of both. The Carter presidency saw the pendulum swing both ways—from a high degree of amity and shared aspirations in the initial years to considerable tension on both sides toward the end. The dividing line was, in almost all respects, sharp: the USSR's invasion of Afghanistan in late December 1979. Running through both periods was discordance between the two countries' nuclear policies, centering in particular on U.S. fuel owed to India's Tarapur nuclear power reactors and obstacles to that supply posed by a treaty-toughened U.S. stance on nuclear exports.