Takes on the question of how NATO, having successfully kept the peace in Europe in the twentieth century, can adapt to the challenges of the twenty-first. Contends that NATO retains value for the United States and Europe, but that it must expand its vision of collective defense in order to remain relevant and effective. This means recognizing the full range of threats that confront NATO members today and affirming that the alliance will respond collectively to an act (whether by an outside state or a nonstate entity) that imperils the political or economic security or territorial integrity of a member state. NATO has been a cornerstone of security in Europe, and of U.S. foreign policy, for six decades. But its ability to continue playing such a central role is unclear. This report takes a sober look at what the alliance and its members must do to maintain NATO's relevance in the face of today's strategic environment.