What happened
The bipartisan NATO Support Act, authored by Senators Tim Kaine and Marco Rubio and folded into the Fiscal Year 2024 National Defense Authorization Act, passed the Senate 87–13 in 2023 specifically because lawmakers in both parties anticipated exactly this scenario: a president threatening to leave NATO unilaterally. The law is direct — the president "shall not suspend, terminate, denounce, or withdraw the United States" from NATO except with two-thirds Senate approval or a separate act of Congress, and it bars federal funds from being used to attempt withdrawal by any other means.
Trump's April 2026 threat came directly after European NATO allies chose not to join offensive US and Israeli strikes on Iran, which Trump appeared to take as evidence the alliance was not delivering reciprocal loyalty. "I was never swayed by NATO," he said when asked whether the alliance's history factored into his thinking.
Legal scholars are divided on what happens next if he tries. Some believe the 2023 law is a clear, binding constraint that makes any unilateral attempt illegal on its face. Others note the president could attempt to invoke independent constitutional authority over foreign affairs, which would almost certainly trigger a legal challenge — most plausibly from Congress itself, though with Republicans controlling the Senate, the political will to sue a Republican president is far from assured. Either way, this marks the first time a sitting US president has publicly threatened to leave NATO in the alliance's 77-year history.