U.S. President Donald Trump’s decision to substantially reduce the U.S.-ROK combined military exercise should concern South Koreans, but not simply because one exercise will be smaller. Exercises can be adjusted for diplomatic or operational reasons. The more important issue is what the decision tells us about changing American expectations toward its allies and whether South Korea understands that change. Trump’s disappointment with South Korea is understandable. Washington increasingly expects allies to contribute to challenges the United States considers important to international security. His frustration over Seoul’s reluctance to participate in U.S. efforts concerning Iran reflects his broader belief that alliances cannot be one-way arrangements in which America responds to threats facing its allies while those allies remain distant from challenges confronting the United States. But South Korea faces serious security challenges of its own. It confronts a nuclear armed North Korea with increasingly sophisticated missiles, growing cooperation with Russia, and a large conventi