Three days after President Donald Trump took office for his second term, he made headlines by claiming that “denuclearization” with Russia and China is “very possible.” Trump has since reiterated this idea, illustrating his long-held ambition to negotiate a nuclear arms control agreement. The President, however, will face a dilemma as he pursues negotiations: the concessions that adversaries demand as preconditions for arms control will undermine allies’ faith in U.S. security guarantees, potentially leading them to pursue their own nuclear weapons. As a result, there is currently a tradeoff between progress on arms control and nonproliferation. President Trump appears willing to make significant concessions on regional security issues—potentially including support for key partners—to bring adversaries to the negotiating table. Three areas of U.S. policy will influence whether this approach raises the risks of allied proliferation: the administration’s approach to nuclear modernization, extended deterrence, and security concessions. The Previous Approach to Partners, Alliances, and Arms Control Historically, the United States has refused to compromise on the security interests of U.S. allies and partners. Successive administrations of both parties have viewed alliances as critical for maintaining the international order. President Biden was a particularly strong supporter of this policy: In Europe, he repeatedly reaffirmed the United States’ commitment to NATO and supplied nearly $83.4 billion dollars in aid to Ukraine after Russia’s invasion. In the Indo-Pacific, Biden established a Nuclear Consultative Group with South Korea and facilitated advancements in U.S.-ROK-Japan cooperation. Biden also took an assertive approach to Taiwan and broke with the United States’ longstanding policy of strategic ambiguity when he said on three occasions that the United States would come to Taiwan’s defense if China attacked. Past support for Ukraine and Taiwan, however, reinforced perceptions among Russian and Chinese leaders that the United States seeks to contain and weaken their countries. As a result, recent efforts to engage adversaries on arms control have been unsuccessful. President Biden, for example, made early progress by extending New START and launching a series of strategic stability dialogues with Russia, but the Kremlin suspended its participation in New START and rejected subsequent U.S. appeals for engagement on arms control after its invasion of Ukraine. President Biden faced similar obstacles with China: The administration made small strides by resuming strategic stability dialogues, but those talks failed to lead to substantiative progress on arms control and China rejected further discussions in response to U.S. weapons sales to Taiwan. President Trump’s Divergence In contrast to previous administrations, President Trump may be willing to accommodate adversaries’ regional security interests at the expense of partners who he perceives to be free riding on U.S. protection. In Europe, the administration temporarily paused military aid to Ukraine, halted offensive cyber operations against Russia, and signaled openness to lifting sanctions. President Trump’s China policy is still taking shape, but previous comments suggest that he views Taiwan as taking advantage of the United States and may not be as committed to the island’s defense as his predecessor. Trump’s approach to Ukraine and Taiwan may help to overcome some of the obstacles to arms control that the Biden administration faced. Discontinuing U.S. support for Ukraine and lifting sanctions on Russia, for instance, would eliminate a key obstacle to dialogue with the Kremlin. Recent comments from Putin about the need to extend New START—a major reversal of the Kremlin’s position during the Biden administration—suggest that Trump’s approach may lead to new engagement. This strategy, however, carries risks for core U.S. security interests. U.S. allies that rely on extended deterrence may see fading support for non-treaty partners—like Ukraine and Taiwan—as a bellwether for U.S. resolve, despite that the United States has no formal obligation to defend these states. To the extent that the administration’s pursuit of deals with Russia and China entails abandoning Ukraine and Taiwan, it will undermine allies’ confidence in U.S. security assurances and may prompt allies to pursue indigenous nuclear programs. What to Watch Three elements of U.S. policy will determine whether Trump’s approach will raise the risks of allied proliferation. As experts and allies monitor this risk, they should watch the trajectory of U.S. nuclear modernization, the administration’s signaling to allies about extended deterrence, and Trump’s concessions to adversaries on regional security architectures. First, Trump’s approach to U.S. nuclear modernization has the potential to build or undermine confidence in U.S. guarantees. President Trump has consistently expressed skepticism about the need to modernize the nuclear force, recently saying that “there’s no reason for us to be building brand new nuclear weapons.” Experts, however, have expressed concern that the current nuclear force is no longer sufficient to deter war with multiple nuclear peers, given its age and limited range of capabilities. Deviation from existing modernization plans could diminish U.S. leverage in arms control negotiations and leave the United States with a force that is ill-equipped to confront the threat posed by adversaries. If allies perceive that U.S. nuclear capabilities are insufficient to deter aggression, the risks of nuclear proliferation could rise. Second, the administration’s signaling to allies about its commitment to extended nuclear deterrence will be a key determinant of allies’ anxiety. Past comments from President Trump about the need for allies to contribute more to their own defense raised concerns about the United States’ commitment to its security obligations. If administration officials continue to suggest that U.S. security guarantees are conditional on defense spending levels or other similar criteria, allies will increasingly question the value of U.S. extended deterrence. Alternatively, if the Trump administration makes clear that, despite changes in force posture and burden sharing, the U.S. remains steadfastly committed to extended nuclear deterrence, it can minimize allied concerns and lessen the risk of proliferation. Third and finally, Trump’s willingness to accommodate adversaries’ regional security interests will shape opportunities for arms control and the risks of proliferation. If, for example, the Trump administration makes broader concessions on security arrangements in Europe during peace talks in Ukraine, allies will grow increasingly concerned about the United States’ commitment to their defense. In particular, the degree in which Trump accommodates Putin’s requests for limits on NATO/U.S. presence in Eastern Europe would significantly undermine allies’ confidence in the United States’ resolve to defend them. Historically, the United States has consulted closely with allies when pursuing strategic arms control agreements. The Trump administration, however, has signaled that it will take a different approach. The administration’s commitment to U.S. nuclear modernization, its signaling about the strength of U.S. extended deterrence guarantees, and its willingness to accommodate adversaries’ security interests will determine the likelihood that this approach prompts U.S. allies to pursue independent nuclear weapons programs.