Europe Faces Uncertainty as New START Ends

European leaders are beginning to recognize that the United States may be unlikely to prioritize European security as it did during the Cold War and its immediate aftermath. Following U.S. President Donald Trump’s escalating threats of aggression toward Greenland, European Union foreign policy chief Kaja Kallas said last month that Europe must adapt to new realities and that “NATO needs to become more European to maintain its strength. The expiration of the final bilateral nuclear arms control agreement between the United States, which serves as NATO’s nuclear guarantor, and Russia, Europe’s principal security threat, leaves European countries in a precarious security position. Some European leaders are considering whether this new uncertainty requires unprecedented action.

The New Strategic Arms Reduction Treaty (New START), the last major U.S.-Russia nuclear arms control agreement, expired February 5, 2026. The treaty limited the number of nuclear warheads each country could deploy on strategic systems to 1,550. Although Russia and the United States were reportedly conducting last-minute negotiations to informally abide by the numerical limits after the treaty’s expiration, Trump renewed uncertainty about the agreement’s future when he posted on social media that New START was a “badly negotiated deal” and that the United States should “work on a new, improved, and modernized Treaty. Barring an agreement, some experts predict that in the coming decade, the U.S. and Russian nuclear arsenals could grow to as many as 3,500 and 2,600 deployed strategic warheads,respectively. Despite not being parties to the agreement, European leaders consider New START a “crucial contribution to international and European security.”

Europe’s concerns

With New START ending, Europe’s primary concerns are:

Curtailed communication. Arms control agreements provide a backbone of stability which can help avert U.S.-Russia conflict. New START included protections against the targeting or disrupting of “national technical means”—U.S. and Russian capabilities for observing one another’s nuclear forces and for providing early warning of potential incoming attacks. This ensured that each side could monitor the other, preventing accidents and misunderstandings that could have catastrophic consequences. The agreement also established a Bilateral Consultative Commission, wherein the United States and Russia could resolve potential conflicts or disagreements about treaty compliance. Without the risk reduction and confidence-building measures of the New START agreement, a nuclear arms buildup and more uncertainty will likely follow its expiration. As consultative mechanisms between the United States and Russia disappear, confidence that the two nuclear superpowers are not embroiled in an arms race will likely wane. While scholars debate the dangers of arms racing, many Europeans fear that without the stabilizing guardrails of a treaty, unrestricted nuclear deployments would create instability across Europe and beyond.

Credibility of extended deterrence. The end of New START coincides with decreasing global confidence that the United States will continue its policy of extended deterrence—the promise to use nuclear weapons to defend NATO and Asian allies from nuclear attack. The past year of the Trump administration has raised significant doubt in Europe about the credibility of these promises. The administration’s National Defense Strategy, which did not explicitly mention extended deterrence, deepened those concerns. Trump’s threats against Greenland and other European allies, his frequently shifting stance on Russia and Ukraine, the circulation of internal Pentagon discussions, and a National Security Strategy that disparages Europe and the transatlantic alliance all fuel European leaders’ fears that the U.S. nuclear umbrella is flimsier than before. The potential for proliferation is not limited to Europe. As New START ends and the broader nuclear nonproliferation order crumbles, Japan, South Korea, and others might see a global arms race as cause to pursue their own weapons. But without a credible nuclear umbrella, nonnuclear European countries like Estonia, Germany, and Poland are particularly vulnerable to aggression from Russia and others.

Europe’s options

With New START’s end, European leaders need to balance a quiet campaign to extend the treaty with the pursuit of practical steps for their post-treaty security. Few European states have endorsed Russian President Vladimir Putin’s proposal to extend New START by one year or offered alternatives, and leaders are hesitant to publicly break from the U.S. stance. “[This] is a very delicate moment,” Astrid Chevreuil, visiting fellow in the Europe, Russia, and Eurasia Program at the Center for Strategic and International Studies, told CFR. “[European countries] don’t want to be seen paving the way for a Russian proposal.” Regardless, most European leaders still view a limited form of arms control as better than none, she said. 

Overall, there is a consensus among European countries that New START has “worked in [Europe’s] favor,”Oliver Meier, policy and research director at the European Leadership Network, told CFR. The former head of the British Armed Forces, Admiral Sir Tony Radakin, decried [PDF] the unraveling of nuclear agreements last year as “one of the most dangerous aspects of our current global security architecture.” Meier added that Russia’s war in Ukraine has deepened most European leaders’ preference to see a continued commitment by the United States and Russia to limit nuclear deployments rather than no agreement at all.

In the face of the increasing unreliability of U.S. security commitments, Europeans are preparing to address their own nuclear security. The likely expansion of the U.S., Chinese, and Russian nuclear arsenals following the treaty’s expiration could pressure the United Kingdom (UK) and France to grow their nuclear forces. Doubts of U.S. nuclear credibility at least partly explain the rationale behind the Northwood Declaration, adopted in July 2025, which deepened nuclear cooperation between France and the UK. Recently, French President Emmanuel Macron has renewed discussions about a European nuclear deterrent, although establishing a French-British nuclear umbrella faces several hurdles. Among the challenges would be convincing Russia and other nuclear adversaries that France and the UK would be willing to risk their own survival by responding with nuclear weapons to Russia aggression against allies like Poland. Both countries have small arsenals—290 for France and 225 for the UK—that are not designed for counterforce [PDF] or damage limitation [PDF]. Even if they overcome these credibility questions, in the short term, France and the UK cannot currently offer Europe the same security guarantees as the United States.

For the nonnuclear states in Europe, U.S. unreliability has shifted long-held opposition to talks of a European nuclear deterrent. The Northwood Declaration opened the door to extending a European nuclear umbrella to select states while upholding their commitments under the Treaty on the Non-Proliferation of Nuclear Weapons. German Chancellor Friedrich Merz has discussed the possibility of Germany cooperating with France under a new nuclear umbrella. Swedish Prime Minister Ulf Kristersson has also reportedly opened preliminary discussions with the French and British about a nuclear agreement. Polish Prime Minister Donald Tusk went further by hinting that Poland might consider developing its own nuclear weapons, though such a move would be costly and potentially dangerous. Russia would almost certainly respond with hostility to any European proliferation, while other states might view the spread of nuclear weapons as an incentive to acquire their own. Even if Europeans initially only proceed with deepened nuclear cooperation, these agreements might “prove to be way stations to nuclear proliferation,” CFR Senior Fellows Erin Dumbacher and Rebecca Lissner wrote for Foreign Policy.  

The end of New START coincides with growing threats to European sovereignty and security, both from Russia and the United States. The treaty’s expiration could heighten U.S.-Russia tensions and increase the risk of miscalculation and misunderstanding, even if Putin and Trump reach a last-minute deal to continue observing numerical limits. Europe has little leverage over the leading nuclear powers to compel them to negotiate. If they no longer have a reliable ally in the United States, the collapse of nuclear arms control creates another concern for Europeans. They might take matters into their own hands to improve their security in novel ways.