NATO, the most successful alliance in history, is beset by unresolved tensions and divergent interests that are undermining its cohesion, credibility and capability. In this new book, Mark Webber, James Sperling and Martin Smith explore four key post-Cold War developments that threaten NATO's survival: an overextended geostrategic reach and an unwieldly security policy portfolio; a failure to address capability short-falls and meet defence spending benchmarks; US weariness and European wariness that call NATO into question; and intra-alliance discord over Russia's place in the European security order and how to deal with Moscow's destabilization of Georgia and Ukraine. The authors propose in response a range of policy options that could reinvigorate NATO, but conclude with a note of caution. Alliances come and go and most are cast into the dustbin of history. If NATO is to avoid this fate, it must not only address the major problems that trouble it, but also get to grips with future challenges to alliance cohesion and credibility, from Brexit to the emerging contest with China.