This year marks the seventieth anniversary of the North Atlantic Treaty Organization, and to say that the celebrations will likely be subdued is an understatement. Co-­founded by Canada in 1949, in the aftermath of the Second World War, the transatlantic alliance has grown from a membership of just twelve countries to twenty-­nine following Montenegro's entry in 2017. NATO's allure — the reason it attracts such a diverse membership, including Iceland (which has no standing army) — comes from being the world's pre-­eminent military alliance, anchored by the military and economic might of the United States. Today, more countries seek to join it, like Ukraine, while others seek tighter bonds, as Brazil has recently. Yet, despite its successes, NATO currently confronts a litany of major challenges.