U.K. Defense Secretary John Healey resigned Thursday after clashing with Prime Minister Keir Starmer's government over military spending, dealing the British leader a setback weeks before a critical NATO summit to include President Donald Trump.

Healey's departure stemmed from a dispute over the delayed Defense Investment Plan (DIP) — the government's long-promised roadmap for military investment and readiness — and as NATO allies face renewed pressure from Trump to boost defense spending.

"John Healey’s resignation is a seismic moment for the government and the Ministry of Defense," Royal United Services Institute (RUSI) Senior Associate Fellow political headaches in terms of a replacement, and trying to get the Defense Investment Plan published."

Healey had been in intense, late-stage negotiations with Starmer and Chancellor of the Exchequer Rachel Reeves over the scale and timelines of the DIP.

Starmer reportedly refused to set out a timeline to reach 3.5% of gross domestic product (GDP) on defense by 2035 — a promise he made to Trump at last year's NATO summit — and would not commit to a firm date for reaching 3%.

Instead, Starmer offered Healey a deal to spend 2.68% of GDP on defense by 2030, up only marginally from 2.6% next year, increased pressure on European allies to fulfill their defense obligations. Trump has frequently criticized NATO alliance members as "free riders."

On June 3, Secretary of State Marco Rubio also told the House Foreign Affairs Committee that the upcoming Ankara summit would be the "most important meeting" in NATO’s history because there are some things "that need to be cleared up and fixed."

He added, "The United States is still in the NATO alliance, and we'll be there."

However, U.S. officials have made it clear that patience is wearing thin.

"Ahead of next month’s NATO summit, POTUS has been clear: Allies must fulfill their commitment to spending 5% of GDP on defense," U.S. Ambassador to NATO Matthew Whitaker posted on X this week.

Furthermore, a U.S. official noted that a U.K. funding package far lower than 18 billion pounds ($23 billion) would send a highly "negative" signal to Trump ahead of the Ankara meeting, according to Germany plans to spend 3.7% of its GDP on defense by 2030.

"Healey knows the threats we face, he knows the capabilities and shortfalls the armed forces have, and if he believes that the financial settlement is not enough to keep the country safe — to the extent that he cannot honorably stay in post — then we are in trouble," Rowlands added.

"While the impact will mainly be felt on Whitehall, the international implications are severe with a NATO summit just three weeks away," Arnold noted.