Alliance politics is a regular headline grabber. When a possible military crisis involving Russia, North Korea, or China rears its head, leaders and citizens alike raise concerns over the willingness of US allies to stand together. As rival powers have tightened their security cooperation, the United States has stepped up demands that its allies increase their defense spending and contribute more to military operations in the Middle East and elsewhere. The prospect of former President Donald Trump unilaterally ending alliances alarmed longstanding partners, even as NATO was welcoming new members into its ranks. Military Alliances in the Twenty-First Century is the first book to explore fully the politics that shape these security arrangements – from their initial formation through the various challenges that test them and, sometimes, lead to their demise. Across six thematic chapters, Alexander Lanoszka challenges conventional wisdom that has dominated our understanding of how military alliances have operated historically and into the present. Although military alliances today may seem uniquely hobbled by their internal difficulties, Lanoszka argues that they are in fact, by their very nature, prone to dysfunction.