Trump called their lack of support “a stain on the alliance that will never disappear,” while German Chancellor Friedrich Merz described the crisis as “a trans-Atlantic stress test.”

“There will be no return to business as usual in NATO,” said Jim Townsend, a former US defence official. “We are closer to a break than we have ever been.”

Trump cannot withdraw the US from NATO without congressional approval, but he could reduce troop deployments and military coordination with allies. The US currently has about 84,000 troops in Europe.

Officials warn such moves could weaken the alliance even without a formal withdrawal.

“He doesn’t need to leave NATO to undermine it,” said former NATO ambassador Stefano Stefanini.

European defence spending has risen, but analysts say major gaps remain in key military capabilities, with some estimates suggesting it could take over a decade and cost around $1 trillion to close them.

Some experts say NATO could survive in a reduced form even without full US participation, while others warn Russia could regain offensive capability against NATO by 2027–2029.

NATO was founded during the Cold War and has since evolved into a major transatlantic security alliance.