When the North Atlantic Treaty Organization was formed just four years after the United Nations, it provided its members with a measure of security in the face of the Soviet Union’s veto power in the senior organization’s Security Council, as well as a means of coping with Communist expansion. Ever since then, the two institutions have been competitors in maintaining peace in the postwar world. Occasionally they have cooperated; more often they have not. In NATO and the UN, Lawrence Kaplan, one of the leading experts on NATO, examines the intimate and often contentious relations between the two and describes how this relationship has changed over the course of two generations. Kaplan documents the many interactions between them throughout their interconnected history, focusing on the major flashpoints where either NATO clashed with UN leadership, the United States and the Soviet Union confronted each other directly, or fissures within the Atlantic alliance were dramatized in UN sessions. He draws on the organizations’ records as well as unpublished files from the National Archives and its counterparts in Britain, France, and Germany to provide the best account yet of working relations between the two organizations. By examining their complex connection with regard to such conflicts as the Balkan wars, Kaplan enhances our understanding of both institutions. Crisis management has been a source of conflict between the two in the past but has also served as an incentive for collaboration, and Kaplan shows how this peculiar but persistent relationship has functioned. Although the Cold War years are gone, the UN remains the setting where NATO problems have played out, as they have in Iraq during recent decades. And it is to NATO that the UN has turned for military power to face crises in the Balkans, Middle East, and South Asia. Kaplan stresses the importance of both organizations in the twenty-first century, recognizing their potential to advance global peace and security while showing how their tangled history explains the obstacles that stand in the way. His work offers significant findings that will especially impact our understanding of NATO while filling a sizable gap in our understanding of post-World War II diplomacy.