Recent refusals by European countries to participate in US and Israeli attacks against Iran have triggered fresh tensions in US-Europe relations. US President Donald Trump, in interviews and social media posts, has repeatedly expressed dissatisfaction with European partners and even threatened to withdraw the United States from the long-standing military alliance, the North Atlantic Treaty Organization (NATO). Within this alliance, the US accounts for a disproportionately large share of defense spending and advanced military technology. If the US were to withdraw, would European countries be able to demonstrate credible collective security capabilities?
What is NATO and when was it established? NATO was founded in 1949 by 12 countries, including the United States, the United Kingdom, Canada, and France. The alliance commits member states to collectively defend any one member that is attacked. Its original purpose was to prevent the expansion of the Soviet Union—today’s Russia—and its communist republics’ sphere of influence in Europe. After the dissolution of the Soviet Union in 1991, many Eastern European countries joined the alliance, including Albania, Bulgaria, Hungary, Poland, the Czech Republic, Slovakia, Romania, Lithuania, Latvia, and Estonia.
Sweden and Finland, which had remained neutral for decades, applied to join NATO in 2022 following Russia’s invasion of Ukraine. Today, the military alliance has 32 member states across Europe and North America. NATO does not have its own standing army but can undertake collective military operations during international crises with contributions from member states. NATO conducted its first military operation in 1994, when US fighter jets shot down Serbian planes bombing Bosnia, in an operation called ‘Operation Deny Flight.’
Ukraine’s application to join NATO is currently under consideration. NATO has not deployed its troops to Ukraine’s battlefield due to fears of engaging in a conflict with nuclear power Russia and has not declared a no-fly zone over Ukraine. However, member states have been supplying Ukraine with weapons and military materials. NATO is a powerful group because of its combined European defense capabilities and the US role, but experts warn that without American involvement, its operational capacity would be significantly weakened. As of December 2025, approximately 68,000 US troops are permanently stationed across 31 military districts and 19 military bases.
“Today, American commitment to NATO is indispensable for both conventional and nuclear military capabilities,” said Dr. Tore Towsig, Director of the Transatlantic Security Initiative at the US-based Atlantic Council. In an email interview, she explained that without US participation, Europe would struggle to maintain crucial capabilities such as long-range military and logistical transport, intelligence, surveillance, reconnaissance, and missile defense systems.
Dr. Gavin Hall, an international security lecturer at the University of Strathclyde in Glasgow, agreed that US withdrawal would be a major blow to European security. “The US has the largest capabilities among NATO members,” Hall told the BBC. European countries understand well that early-stage military deployment and operations heavily rely on the US. Nonetheless, NATO members have maintained significant military capacity independently. “NATO missions such as air patrols over the Balkans and maritime patrols in the Mediterranean continue even without direct US involvement.”
Trump harbors deep distrust of NATO’s inability to compel other member states to increase defense spending. NATO set a requirement in 2014 for all members to spend at least 2% of their Gross Domestic Product (GDP) on defense, aiming to increase it to 5% by 2035. According to the Stockholm International Peace Research Institute, the US accounted for 66% of NATO countries’ combined defense expenditure in 2024. When asked why European countries are expected to increase their defense budgets, Towsig noted that “the US covers 15% of NATO’s shared budget needed to operate headquarters, military commands, and structures. If partners increase defense spending and the US cuts funding, reliable sources of funding will have to be found.”
NATO commanders devised detailed plans in 2013 to counter potential Russian attacks in the Arctic, North Atlantic, Central Europe, and the Mediterranean. Last year, they announced plans to increase forces on high alert from 40,000 to 300,000 troops. Every three years, NATO conducts a military exercise called Steadfast Defender, involving 90,000 troops from all 32 countries, with the next exercise scheduled for 2027. “While NATO sometimes has limited capabilities, clear leadership and guidance are essential,” Hall said. He emphasized the need to improve NATO’s nuclear capacity. France and the UK possess nuclear weapons but in far smaller numbers compared to Russia. European countries are preparing plans for early defense and rapid deployment, but Hall remains cautiously uncertain about long-term sustainability.
To develop combat skills and enhance collective combat readiness, the US military regularly conducts bilateral and multilateral exercises with European countries. NATO membership facilitates US access to numerous European military districts. “Without NATO, the US would need to negotiate separate defense cooperation agreements with individual countries,” Towsig explained. While the US has varying agreements with some European states, new agreements would be necessary with others. If Trump follows through on his threat to exit NATO, it would have profound military and political consequences. “A US split from NATO would have devastating effects on US-Europe unity and NATO’s ability to resist Russia and other adversaries,” Towsig warned. NATO has also fostered significant goodwill for the US in Europe beyond security guarantees. “It’s important to remember that NATO’s collective defense provision, Article 5, has been invoked only once—for the US’s defense following the September 11, 2001 terrorist attacks.”
Hall suspects Trump’s threats to leave NATO might be an attempt to leverage issues like Greenland—whom Trump suggested the US should acquire for national security—or trade. A US departure would be detrimental for both sides. “It would undermine the Euro-Atlantic security consensus established after World War II. The message would be that the US is no longer invested in European security and any changes in Europe’s security arrangements would not affect US interests.”
