In its first month in office, the administration of President Donald Trump has taken steps that could upend America’s alliance system. Leaders and officials in allied capitals in Europe and Asia now rightly wonder whether Trump would follow through on treaty commitments to come to their defense if they were attacked. An obvious and unsurprising consequence of this U.S. policy reversal, given the threat of aggression from Russia, China, and North Korea, is growing interest among U.S. allies in the protection of nuclear weapons.

For example, shortly before his party prevailed in Germany’s national election on February 23, incoming chancellor Friedrich Merz stated in an interview, “We must prepare for the possibility that Donald Trump will no longer uphold NATO’s mutual defense commitment unconditionally.” He argued that “We need to have discussions with both the British and the French—the two European nuclear powers—about whether nuclear sharing, or at least nuclear security from the U.K. and France, could also apply to us.” Separately, on February 25, South Korean Foreign Minister Cho Tae-yul argued in a National Assembly session that “it’s premature to talk about [nuclear weapons as a] Plan B, but that doesn’t mean it’s off the table.” He added, “Given that international situations are developing in unpredictable directions, this is a principled response that we must prepare for all possible scenarios.”

As these statements portend, if the Trump administration proceeds to dismantle the U.S. alliance system, more states—whether U.S. allies or adversaries—likely will seek nuclear weapons. This could have catastrophic effects on the ability of future administrations to prevent conflict, stop the further spread of nuclear weapons, and sustain the capacity to contain Chinese aggression.

Questioning Trump Nominees

In the coming weeks, several individuals nominated for senior positions at the State and Defense Departments and National Nuclear Security Administration (NNSA) will face Senate confirmation hearings. These nominees include Thomas DiNanno to be undersecretary of state for arms control and international security, Elbridge Colby to be undersecretary of defense for policy, and Brandon Williams to be NNSA administrator.

These individuals and subsequent nominees for assistant secretary–level positions at relevant agencies (including the Commerce and Treasury Departments) will be charged with managing the U.S. alliance system and maintaining U.S. programs—including financial tools and export controls—to prevent the spread of nuclear weapons. This assumes that these departments and the expert government staff needed to perform these functions survive the Department of Government Efficiency’s “wood chipper.”

The confirmation process offers an important opportunity for members of Congress to press these individuals on the potential implications of the Trump administration’s alliance policies for U.S. national security, especially with regard to threats of nuclear war and the spread of nuclear weapons.

Focus on Asia

U.S. security alliances with South Korea and Japan, through which the United States extends nuclear deterrence (the so-called nuclear umbrella) and positions military forces in the region, are critical to protecting U.S. security and economic interests in East Asia. These alliances have also prevented both Seoul and Tokyo from seeking nuclear weapons, another pillar of bipartisan U.S. national security policy for more than sixty years. With an increasingly assertive China and growing nuclear threats from North Korea, these alliances remain an important force multiplier for the United States. 

Colby has argued for policies that could significantly alter U.S. alliance commitments, encourage the spread of nuclear weapons, and ultimately weaken U.S. power relative to China. In a May 2024 interview with Chosun Ilbo, for instance, he offered, “North Korea is clearly the biggest threat to South Korea, and I don’t think it’s right to rule out the possibility of South Korea arming itself with nuclear weapons as North Korea advances its nuclear capabilities.” He added, “It’s time to put all options, including South Korea’s own nuclear armament, on the table and discuss them.” He appears to believe that nuclear armament by East Asian allies and an abandonment of the U.S. extended deterrent could help the United States reduce its defense burden and concentrate its force posture on containing China.

Whereas Colby has frequently made such arguments, other nominees’ views on issues of nuclear proliferation and U.S. alliances are less well known. However, DiNanno is clearly a skeptic of some international nuclear institutions.

What Congress Should Ask

Given the potential for a major departure from past practice on these issues, members of Congress should query nominees for relevant national security positions on the potential harmful implications for the United States. Below are several key issues and questions worthy of debate. 

Sustaining U.S. geopolitical power: Any additional state that acquires nuclear weapons dilutes U.S. national power. The presence of nuclear weapons in key regions inherently increases risks to U.S. military operations; this creates a deterrence effect that diminishes the comparative advantages of maintaining preponderant conventional military capability. Further, it complicates the U.S. ability to manage the potential for conflict and conflict escalation. This is true regardless of whether the proliferator is a U.S. ally, partner, or adversary. Driven by that understanding, the United States has extended deterrence to allies both as a means of preventing conflict that would harm U.S. interests and to incentivize allies not to seek nuclear weapons.  

• How does a policy encouraging nuclear weapons acquisition by another state put America first and prioritize U.S. security?
• How would abandoning extended deterrence with South Korea increase the security of the United States? What impacts would there be in other regions of strategic and economic importance to the United States, namely Europe and the Middle East?
• Are these regions still important to the U.S. economy?
• How would the United States’s withdrawal from its commitments impact our ability to compete with China in these regions? Would it harm opportunities for U.S. businesses?
• If the United States withdraws its extended deterrence protection from South Korea, would we continue to expect South Korea (or Japan, or any other U.S. ally) to help in a conflict with China? 

Risk of conflict with North Korea and China: If South Korea pursues nuclear weapons with U.S. support or tolerance, the risk of conflict with North Korea and/or China could actually increase in the near term. In the years it would take South Korea to deploy a survivable nuclear force, both North Korea and China would have strong incentives to disrupt or destroy its nuclear program militarily or through other actions that could easily escalate to conflict. 

• Would you advocate sustaining the U.S. nuclear umbrella over South Korea until it has a survivable nuclear force?
• Would the United States militarily defend South Korea if it were attacked while in the process of acquiring the nuclear weapons? Would this implicate the United States in targeted aggression from North Korea and China?
• How would you suggest the United States protect its forces in the region (regardless of whether they are still on the Korean Peninsula), which could become targets of a North Korean or Chinese attack?  

Further, if South Korea succeeded in acquiring a survivable nuclear force, it could be tempted to take provocative actions intended to force North Korean capitulation or topple the Kim Jong Un regime. This is not far-fetched: it appears that prior to his aborted martial law attempt in December 2024, South Korean President Yoon Suk-yeol’s administration on several occasions took or planned actions intended to provoke a North Korean military response that would have permitted Yoon to suspend the constitution.  

• How would the United States obtain assurances that a nuclear-armed South Korea would act in ways consistent with U.S. interests to avoid regional conflicts, and mitigate the risk of nuclear war that could spill over to the U.S. homeland?
• Given the current political turmoil in South Korea, during which it has not always been clear who is leading the country, would you trust South Korea to forever look out for U.S. interests if it acquired nuclear weapons? 

Stopping future nuclear weapons proliferation: South Korean interest in nuclear weapons is understandable in light of the growing threats it faces and questions about the reliability of U.S. security commitments. However, encouraging or tolerating nuclear armament by any U.S. ally would reverse decades of efforts to prevent the spread of nuclear weapons. These efforts have included maintaining the international system that enables monitoring and early warning of nuclear proliferation and facilitating peaceful nuclear energy trade as part of the grand bargain for states not pursuing nuclear weapons. This system of treaties and institutions has been remarkably successful and a long-standing pillar of U.S. national security.

• Should the United States assist South Korea or any other ally in building nuclear weapons, or should it look the other way?
• Would U.S. tolerance of, or even assistance to, South Korean nuclear armament signal U.S. abandonment of the system that has prevented widespread proliferation since 1968?
• If South Korea pursues a nuclear weapons program without consequence from the United States and other countries subsequently seek to do the same, how will you respond?  What would you say to Iran if it followed the same path?
• Given that U.S. anti-proliferation policies provide the backbone for enabling nuclear energy trade, how would that set back U.S. efforts to strengthen the competitiveness of American nuclear industry in overseas energy markets?

Sorting through the potential implications for U.S. national security, U.S. forces deployed overseas, and the American economy is vital before signing on to major changes in policy toward allies and nuclear proliferation. Senate committees must explore nominees’ views on these issues and challenge simplistic promises of reduced defense burden and lower risks to U.S. interests.