For decades, the global nuclear security order rested on a simple premise: allies of the United States could forgo their own atomic arsenals in exchange for protection under Washington's nuclear umbrella. That foundation is now showing cracks, as the Trump administration's latest defense strategy de-emphasizes extended deterrence and presses allies to shoulder more of their own security burden.

The newly released 2026 National Defense Strategy notably omits explicit language on extended deterrence while demanding allies increase their defense spending. According to Ankit Panda, a senior fellow in the Nuclear Policy Program at the Carnegie Endowment for International Peace, allied nations increasingly perceive Washington as reluctant and unreliable, prompting them to explore various forms of strategic autonomy. This shift is manifesting in concrete policy changes across South Korea, Japan, and parts of Europe.

President Trump recently ordered a reduction in joint military exercises with South Korea, describing them as sending a "completely inappropriate and hostile" signal to Pyongyang. Simultaneously, Washington has engaged in negotiations with Seoul over uranium enrichment and nuclear fuel reprocessing for peaceful purposes, while also signing a civilian nuclear agreement with Saudi Arabia that reportedly could pave the way for the kingdom's domestic enrichment capabilities.

In March, French President Emmanuel Macron announced an increase in France's nuclear warhead inventory—the first expansion since 1992—and proposed extending French nuclear protection to other European nations through a "forward deterrence" partnership. Nine countries, including Germany and Greece, have already joined this initiative. Japan has also intensified its nuclear defense discussions, with Prime Minister Takaichi Sanae declining to rule out revising the country's three non-nuclear principles, fueling speculation about Tokyo's long-standing commitment against hosting nuclear weapons. Meanwhile, Lithuania and Finland, both bordering Russia, have moved closer to lifting their long-standing domestic bans on nuclear weapon deployment.

None of these developments signal an immediate decision by any of these nations to build nuclear arms, yet the shifting security landscape could trigger a cascade of consequences. Shounak Set, a researcher at Singapore's S. Rajaratnam School of International Studies, notes that actions taken by one state to bolster its own security can inadvertently make adversaries feel less secure, thereby increasing their incentives for preemptive strikes. Andrew Facini, a senior fellow at the Center for Strategic Risks, warns that deploying a nuclear deterrent inevitably intensifies tensions and forces opponents to respond, potentially accelerating the very security risks a nation sought to avoid.

Iran stands as the most immediate case study of these dynamics. Tehran's uranium enrichment program partially contributed to the current Middle East conflict, and the Trump administration repeatedly stressed that Iran must not be allowed to acquire nuclear weapons, culminating in the 2025 joint U.S.-Israeli strikes targeting this objective. Alexandra Bell, president and CEO of the Bulletin of the Atomic Scientists, points out that while Iran has not been confirmed to possess nuclear weapons, the war has introduced a troubling uncertainty: whether attacking Iran may have pushed its leadership toward a final decision to actually pursue nuclear armament—a choice they had not yet made.

The risks are further amplified by ongoing arms expansions among nuclear-armed states. A June report from the Stockholm International Peace Research Institute (SIPRI) indicates that the United States and Russia together hold approximately 83% of the world's operational warheads. Hans Kristensen, a senior fellow at SIPRI, observes that nations seeking nuclear weapons are generating new risks and fueling an arms race. Globally, more than 12,000 nuclear warheads exist, with over 9,000 in military-ready stockpiles, approximately 4,012 deployed on missiles and aircraft, and up to 2,200 on high alert, capable of launching within minutes.

A September report from the Carnegie Endowment warned that if additional friendly, hostile, or even sub-state actors acquire nuclear capabilities, the world would become more dangerous for the United States and all other nations. Facini describes the proliferation cascade as "an ever-compounding risk," where each additional nuclear state increases the probability of miscalculation, compresses decision-making timelines, and sparks regional arms races, potentially elevating even conventional conflicts to existential stakes.

United Nations Secretary-General António Guterres warned in May that the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) is being "eroded." The treaty, which entered into force in 1970 and was extended indefinitely in 1995, rests on a core bargain: non-nuclear states commit to forgoing nuclear weapons, while nuclear states share peaceful nuclear technology and pursue disarmament. Bell asserts that the treaty has broadly curbed large-scale proliferation, but she cautions that if its safeguards are dismantled, nuclear materials, facilities, and expertise would circulate more widely, heightening the risk of nuclear terrorism. She urges Washington to reduce the incentives driving other nations toward nuclear arms, warning: "Building things is hard, but breaking things is easy. If we break the NPT, we may never recover from that security gap."