NATO leaders gather in Ankara to tackle US-Europe tensions, defence gaps and support for Ukraine’s military resilience.

The war in Ukraine and the fall out from the Iran conflict are high on the NATO summit’s agenda this week. But well before Secretary General Mark Rutte takes to the stage on Tuesday, NATO’s most powerful individual had already set the tone.

“Ridiculous for the U.S.A. to continue along this one sided path when the relationship is not reciprocal,” US President Donald Trump wrote late last week. “They were not there for us!!!”

As 32 leaders of the transatlantic body gather in Ankara for their two-day summit starting on Tuesday, the frosty relationship between the military alliance’s members and the United States has emerged as the single biggest challenge overshadowing its future.

NATO says the summit will focus on three priorities: increasing defence investment, expanding Europe’s defence industrial base and ensuring long-term military support for Ukraine. The meeting comes after allies pledged last year to spend the equivalent of five percent of GDP on defence, with European allies and Canada increasing defence investment by $139bn in nominal terms in 2025 alone.

But talks will be overshadowed by Trump’s threats to pull the US out of NATO and his plan to move troops and weapons out of Europe. On May 1 , the Pentagon announced the withdrawal of about 5,000 troops from Germany, following a “thorough review of the Department’s force posture in Europe”.

“I don’t think the alliance is at a breaking point,” said Ian Lesser, distinguished fellow at the German Marshall Fund of the United States. “But it is entering a period of profound adjustment.”

Trump’s scepticism towards NATO is not new, but the recent conflict with Iran has deepened tensions within the alliance. He has repeatedly criticised European allies for refusing to back Washington militarily, particularly by declining to participate in efforts to reopen the Strait of Hormuz.

Another major source of friction is military spending. On Thursday, Trump again criticised the defence alliance saying it was “ridiculous” that Washington spends more money on NATO than any other country to protect them “without getting any benefit from doing so”.

Experts say that while the US has changed its posture towards NATO, a US pullout from it is unlikely considering the legal and political challenges in doing so, especially as the country is preparing for key midterm elections which will determine who retains control of Congress.

To formally withdraw, Trump would need a two-thirds majority in the US Senate or an act of Congress – scenarios that are unlikely to come to pass any time soon, with NATO still enjoying broad support among many legislators in both major parties in Washington. “It’s in the US interest to stay engaged in Europe’s security and maintain a key role in NATO – and this is a view shared on both aisles in Washington,” said Lesser.

Europeans have given up on restoring the alliance-based trust of decades past, but hope this summit will be an occasion for some planning, said Sophia Besch, a senior fellow in the Europe Programme at the Carnegie Endowment for International Peace.

“The one thing they still dare hope for is greater predictability. They have accepted that the American commitment to Europe is changing; what they want is a more orderly version of the transition,” said Besch. “The fear behind this is well-founded: A botched handover from a US-led to a Europe-led NATO opens up a deterrence and defence gap.”

Despite concerns over the potential reduction in US support, European allies would not be left defenceless. Russia’s invasion of Ukraine exposed both the fragility of Europe’s defence industrial base and the extent to which many NATO members depend on Washington for critical military capabilities. At the same time, repeated strains in transatlantic relations – from disputes within NATO to Trump’s threats to seize Greenland – have reinforced the push for greater European strategic autonomy. As a result, defence spending among European allies rose by 62 percent between 2020 and 2025.

Yet major capability gaps remain. According to the International Institute for Security Studies (IISS), European countries continue to rely heavily on the US for long-range strike capabilities, intelligence, surveillance and reconnaissance, satellite-based assets, logistics and integrated air and missile defence.

Closing those gaps will be a long-term challenge. The IISS estimates that replacing the most critical US conventional military capabilities would require roughly $1 trillion and could take a decade or more. Europe’s defence industry also faces difficulties expanding production fast enough, while many armed forces continue to struggle with recruitment and retention.