The mood will be somber rather than celebratory when NATO foreign ministers gather Thursday in Washington to commemorate the organization’s 70th birthday. History’s most successful multilateral alliance has had quite a run. A tranquil retirement, however, is not in the cards.

NATO’s 29 members confront a daunting agenda. The alliance is grappling with how to deter Russian aggression against its eastern flank; combat Kremlin-inspired and Kremlin-sponsored cyberattacks and political subversion; address the retreat of democracy in several member states; reinforce a still-fragile peace in the Balkans; manage an orderly withdrawal from Afghanistan; improve burden-sharing; adapt capabilities in light of technological innovation; and craft a joint approach to China’s rise. ...