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 for Europe’s conventional defense and the lead in supporting Ukraine. South Korea is described as capable of taking primary responsibility for deterring North Korea, with “critical but more limited” U.S. support.
According to the NDS, greater South Korean responsibility will grant U.S. forces more flexibility to respond to contingencies elsewhere in the Indo-Pacific. This has raised concerns in Seoul about South Korea’s ability to deter North Korea with less U.S. support.
The burden-sharing and -shifting strategy is part of the Trump administration’s answer to what the NDS calls the “simultaneity problem”: the possibility that U.S. adversaries could act together or exploit crises opportunistically, across several theaters. If allies can maintain favorable balances in their own regions, the logic goes, then Washington can focus its forces where they are most necessary rather than trying to dominate every zone at once.
The strengthening Russia-North Korea relationship exposes a complication with that logic, because one theater is changing the threat in another. North Korea has supplied Russia with large quantities of artillery ammunition and ballistic missiles, and it has sent roughly 15,000 troops to Russia’s Kursk region. Zelensky recently claimed that another 30,000 to 50,000 troops could be deployed to Russia (which North Korean leader Kim Jong Un’s sister, Kim Yo Jong, has since dismissed as “groundless”).
North Korea’s forces have gained experience that they could not acquire at home, operating in a battlefield shaped by drones, electronic warfare, precision strikes, and rapid tactical adaptation. Analysts have already identified signs that North Korean forces have adjusted their tactics and use of unmanned systems based on their experiences in Ukraine.
The Ukrainian battlefield gives Pyongyang something that a test range cannot: repeated, real-world feedback on how its missiles perform under combat conditions, including against Western-supplied air defense. Reuters reported last year that North Korean ballistic missiles fired by Russia had become markedly more accurate than earlier versions.
But although U.S. military officials explicitly cited North Korea’s combat experience in Ukraine as one of the developments that the United States and South Korea needed to account for, Trump still ordered the substantial reduction of the Ulchi exercise, shortening the drills from 11 days to five and scaling down field training. The president’s explanation linked the cut not only to his desire to reduce tensions with Pyongyang, but also to his frustration with Seoul for declining to support U.S. operations involving the Strait of Hormuz—tying a bilateral alliance decision on the Korean Peninsula to U.S. priorities in the Middle East.
Moscow also gives Pyongyang something beyond battlefield experience: a major-power partner willing to sustain a military relationship despite international sanctions and political pressure. Russia’s backing makes North Korea harder to isolate and deter. The two countries have continued to deepen their strategic relationship while concern grows in Washington and Seoul about what Russia may provide North Korea in return for military support.
This creates a problem for Washington’s regional division of labor. South Korea is being asked to take greater responsibility for a North Korean threat, even though that threat is being shaped partly on a battlefield that Washington wants European allies to manage. If Russia were simply creating a crisis in Europe while North Korea created another in Asia, then Washington could at least try to assign primary responsibility for each to different allies. That becomes harder when activity in Europe changes the capabilities, incentives, or operational knowledge of the adversary that South Korea is expected to deter in Asia.
Generally, Seoul accepts that it should carry more responsibility for its own defense. But for Seoul, greater responsibility should not mean a less dependable U.S. role, or that South Korea should automatically inherit security obligations wherever North Korean forces and weapons appear.
South Korea has a genuine security interest in what happens on the Ukrainian battlefield, but acting on that interest would carry political and diplomatic costs. South Korean President Lee Jae-myung’s government is trying to open dialogue with Pyongyang: The president has called for talks aimed at reducing military tensions and eventually replacing the 1953 armistice with a more durable peace regime, and his government treats the containment of North Korea’s nuclear capabilities as part of a longer diplomatic process. Direct military assistance to Ukraine could complicate that effort (although Pyongyang has shown little interest in Lee’s outreach so far, revealing Seoul’s limited diplomatic leverage).
South Korea also has Russia to consider. Before Russia’s full-scale invasion of Ukraine, the two countries had developed a significant commercial relationship. Bilateral trade reached roughly $27 billion in 2021, with Russia supplying energy and raw materials while South Korean companies built substantial businesses. The war, Seoul’s participation in economic sanctions and export controls against Russia, and Russia’s growing military alignment with North Korea have eroded those ties. But Seoul still has reason to avoid treating their destruction as necessarily permanent, particularly if a future political settlement in Europe creates room for economic normalization.
This doesn’t mean that greater allied burden-sharing is unworkable; the United States simply no longer possess the resources—or the political appetite—to shoulder the dominant military burden in every theater. U.S. planning will thus have to cross the boundaries drawn by its own burden-sharing strategies. Intelligence gathered in Ukraine about North Korean missile performance, troop behavior, and battlefield adaptation should feed directly into U.S.-South Korean planning. Seoul’s longtime experience monitoring North Korean capabilities can likewise help Ukraine and European states encountering North Korean weapons and personnel. Washington is particularly well positioned to connect them because it sits at the center of security networks in both Europe and Asia.
But acting as that connector requires predictability and reliability, and erratic behavior from Washington is creating a major vulnerability in cross-theater coordination. South Korea’s foreign minister said that neither Seoul nor U.S. government officials knew in advance that Trump would order the de-escalation of the Ulchi military exercises. Meanwhile, Trump simultaneously opened the possibility of another meeting with Kim Jong Un.
Pyongyang has dismissed Trump’s immediate gestures without ruling out renewed diplomacy. Kim Yo Jong denied that there had been any recent communication between the two leaders and said that reducing U.S.-South Korean military exercises did not change what Pyongyang considers Washington’s hostile policy. Yet she also described the personal relationship between Trump and her brother as “excellent,” leaving open the possibility of future engagement.
Last year’s National Security Strategy describes the United States as the “convener” of a burden-sharing network. That role will become more important, not less, as Washington asks allies to do more. “Convening” cannot just mean assigning Europe the responsibility for Russia and South Korea the responsibility for North Korea. It must mean connecting allies when adversaries move weapons, troops, technology, and military knowledge between those theaters, and maintaining the consultation needed to make those connections credible.
The lesson from the Russia-North Korea partnership is that burden-sharing cannot mean strategic compartmentalization—and that shifting burdens geographically cannot, by itself, solve the simultaneity problem that Washington has identified. Washington may ask Europe to take the lead on Russia and South Korea to take the lead on North Korea. But when its adversaries connect those theaters, the United States will have to connect its allies as well.
