For decades, allies worldwide rested in the comfort of the U.S. nuclear umbrella, confident they didn’t need their own weapons of mass destruction.

That belief is rapidly evaporating as President Donald Trump’s latest National Defense Strategy for 2026 dropped any explicit mention of extended deterrence and has pressed allies to spend more of their own money on defenses.

The change is prompting more governments to reconsider whether to acquire nuclear weapons of their own, raising the risk of a proliferation cascade, analysts say.

Allies almost universally perceive Washington as becoming more “disengaged and unreliable,” prompting “various flavors of strategic autonomy,” said Ankit Panda, senior fellow in the Nuclear Policy Program of the Carnegie Endowment for International Peace.

Most recently, U.S. President Donald Trump ordered a reduction in joint military exercises with ally South Korea, saying that the exercises sent a signal that was “totally inappropriate and hostile” to North Korea.

Earlier last month, the U.S. opened talks on peaceful South Korean enrichment and reprocessing, and signed a Saudi civil-nuclear pact that reportedly could create a path to domestic enrichment. 

In a bid to strengthen Europe’s independence, French President Emmanuel Macron announced in March the country’s first warhead increase since 1992 and promised to extend its nuclear umbrella to other countries in the bloc under the so-called “forward deterrence” partnership. Nine countries, including Germany and Greece, have joined the initiative.

Japan is also doing more in the nuclear defense realm under Prime Minister Sanae Takaichi, whose reluctance to rule out revising Japan’s three non-nuclear principles late last year has drawn scrutiny, fueling speculation that she could shift away from the decades-old commitment to refrain from assuming any role in deployment of nuclear weapons.

Leaders of Lithuania and Finland, both sharing a border with Russia, have moved closer to scrapping the longstanding ban on domestic deployment of nuclear weapons.

None of these developments amounts to a decision to build a bomb — but one state’s efforts to increase its security can leave adversaries feeling less secure, potentially encouraging them to strike first, said Shounak Set, a research fellow at the S. Rajaratnam School of International Studies in Singapore.

“You cannot build or deploy a nuclear deterrent without immediately raising tensions and forcing adversaries to react,” said Andrew Facini, senior fellow at the Council on Strategic Risks, possibly accelerating the security risks a proliferator is trying to avoid.

Defense ministries in the U.S. and Japan, as well as the European External Action Service, the bloc’s diplomatic service office, did not respond to CNBC’s requests for…