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U.S. President Donald Trump’s decision last week to scale down the annual Ulchi Freedom Shield military exercise with South Korea came at an awkward moment, given Ukrainian President Volodymyr Zelensky’s recent warning that North Korea’s military capacity is expanding. U.S. and South Korean military planners had designed this year’s drills to account for expected changes in modern warfare, including drones, GPS disruption, and cyberattacks. But the drills were also intended to help prepare for a North Korea that is gaining invaluable wartime experience from its involvement in Russia’s war with Ukraine.

In the days after Zelensky’s warning—when he also appealed to South Korea for help strengthening Ukraine’s air defenses—Russian forces used North Korean ballistic missiles in a deadly attack on the Ukrainian city of Zaporizhzhia, and Pyongyang launched another ballistic missile from its east coast. Lessons from that battlefield will shape any future military clash that the United States and South Korea might face in Northeast Asia.

U.S. President Donald Trump’s decision last week to scale down the annual Ulchi Freedom Shield military exercise with South Korea came at an awkward moment, given Ukrainian President Volodymyr Zelensky’s recent warning that North Korea’s military capacity is expanding. U.S. and South Korean military planners had designed this year’s drills to account for expected changes in modern warfare, including drones, GPS disruption, and cyberattacks. But the drills were also intended to help prepare for a North Korea that is gaining invaluable wartime experience from its involvement in Russia’s war with Ukraine.

In the days after Zelensky’s warning—when he also appealed to South Korea for help strengthening Ukraine’s air defenses—Russian forces used North Korean ballistic missiles in a deadly attack on the Ukrainian city of Zaporizhzhia, and Pyongyang launched another ballistic missile from its east coast. Lessons from that battlefield will shape any future military clash that the United States and South Korea might face in Northeast Asia.

These developments expose a problem for the Trump administration’s foreign-policy strategy—specifically, its intended division of defense labor among U.S. allies. Washington wants its allies to assume more responsibility for security in their own regions, freeing the United States to concentrate resources on other priorities. But Russia and North Korea are making those regions increasingly difficult to separate, leaving allies such as South Korea to carry more of the burden even as conflicts elsewhere make the threats that they face harder to deter. Washington can regionalize responsibility, but it cannot regionalize threats.

The Trump administration’s 2025 National Security Strategy calls for “burden-sharing and burden-shifting,” requesting that wealthy allies assume primary defense responsibility for their regions so the United States can act as the “convener” and “supporter” of a broader burden-sharing network. The 2026 National Defense Strategy (NDS) further specifies that European allies are expected to take primary responsibility........

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