This article reviews the novel challenges to Northeast Asian security that have arisen since the Russian invasion of Ukraine and related developments, especially the growing military capabilities, foreign policy ambitions, and malign policy coordination of North Korea, Russia, and China. Not only does each authoritarian state present an independent challenge to South Korea, Japan, and the United States, but in collaborating, North Korea, Russia, and China amplify these dangers. In particular, Beijing and Moscow are pressing for curtailment of U.S.- allied cooperation regarding missile defenses, nuclear deterrence guarantees, and missile deployments. They also manifest more tolerance of DPRK provocations, even as these have intensified in frequency and intensity. Despite these challenges to international peace, the remarkable speed and scale of Western democratic solidarity in support of Ukraine and other trends have provided the new South Korean government with opportunities to enhance regional security in cooperation with other powerful democracies.