When asked by President Trump to back the United States and Israel in their war against Iran, European governments responded in strikingly similar terms: they would defend their citizens, bases, partners and shipping lanes, but they would not join an offensive war they had neither chosen nor been consulted on.

European scepticism toward US interventions is not new. Since 1945, many American wars have been seen in Europe as strategically misguided or peripheral to Europe’s own security priorities. The Korean War, launched under President Truman in 1950, was interpreted by many West European governments less as a defence of Korea than as a Soviet ploy to draw western resources into Asia and away from Europe. A similar judgment shaped views of Vietnam: many saw presidents Kennedy and Johnson’s effort to stabilise South Vietnam as a costly dead end that damaged the wider standing of the West. Likewise, while many European governments supported sanctions after Iraq’s invasion of Kuwait in 1990, backing for a large US-led air and ground campaign was more uneven, and the Iraq War of 2003 was even more bitterly contested.

Indeed, scepticism has not necessarily stopped European leaders from playing an assisting role in American-led interventions. Europeans fought in these wars in significant numbers. Even in Vietnam, where no West European state sent combat troops, key governments still provided diplomatic, financial, humanitarian or technical assistance. This long-standing pattern raises a question: how was the tension between European scepticism and support reconciled in the past, and why has that mechanism failed now? Why did European governments back US-led wars from Truman to Biden, yet refuse to do so under Trump over Iran?

Many analysts explain Europe’s refusal as a product of today’s fraught transatlantic relationship. Europeans feel neglected, abused and snubbed. That matters, but the larger story is structural. What is fading is not only trust between governments, but also the hegemony that once underpinned both America’s global role and the Atlantic order. Where European governments once concluded that American backing was indispensable at almost any cost, many are now prepared to risk Washington’s anger. Dependence on the United States, above all in military affairs, has not disappeared, but more European capitals are acting on a once unthinkable assumption: that they must learn to navigate a post-American world.

A glance backwards at European involvement in American wars from Truman to Trump sheds light on the longer-term trends underpinning both scepticism towards, and support for, Washington’s interventions.

In 1950, US Secretary of State Dean Acheson was central to building a multinational coalition for Korea. He never believed the contributions of allied troops could be decisive militarily; rather, their value was political. A broad coalition would show the Soviet Union that the US position in Korea enjoyed international legitimacy. Even so, coalition-building proved difficult.

The British government initially opposed US demands for troop contributions. London supported resisting communist aggression in principle, but feared that the conflict in Korea would drain western resources from Europe. Meanwhile, Britain’s military chiefs argued that their forces lacked both the troops and the money for a ground deployment, preferring instead to offer the British Pacific Fleet under US command. Washington, however, made clear this was insufficient. Acheson pressed London hard, warning that failure to meet US expectations could damage the Anglo-American relationship in ways that went beyond the immediate crisis. British officials understood the implication: refusing troops could jeopardise wider cooperation, including American support for European defence and the broader political relationship. In the end, Britain joined not because military logic compelled it, but because the political cost of refusal seemed too high.

Greek participation in the Korean War reflected a different but related form of dependence. Already reliant on US military and financial assistance in its struggle against communist insurgents, Athens sent an infantry battalion as a signal of gratitude and alignment. Later, Greece and Turkey linked deeper Korean War participation to NATO membership and aid; both entered the alliance in 1952. French and Dutch support likewise reflected their own entanglements in Asian conflicts – France in Indochina and the Netherlands in the Dutch East Indies – and their need for American backing. In each case, dependence on US power was decisive.

The coalition-building effort for Vietnam began in 1964, when President Johnson launched the More Flags Program to enlist partners against North Vietnam. Its purpose was not only military; Johnson also wanted to strengthen the war’s international legitimacy and avoid the appearance of a purely American – even a ‘white man’s club’ – intervention in Asia. Coalition warfare also promised domestic political and financial benefits: allied contributions could reduce the burden on the United States.

In the end, no West European country joined the military coalition. Still, several provided important assistance in other ways. The UK sent a British Advisory Mission to South Vietnam in 1961 to advise on administration, police and counterinsurgency. Prime Minister Harold Wilson later continued to give Washington diplomatic backing, even as he tried to limit escalation and encourage negotiations. For its part, West Germany contributed financial, developmental and humanitarian assistance to South Vietnam from 1955 onward and became its largest West European donor. For Washington, Bonn’s stance on Vietnam was part of a broader test of whether West Germany was behaving as a dependable Cold War ally. Bonn understood the issue in similar terms. Support for South Vietnam was less about Southeast Asia itself than about preserving US backing within the Atlantic alliance.

The administration of George H.W. Bush devoted immense efforts to assembling a broad coalition for the First Gulf War. From James Baker’s private papers, three motives stand out. First, the Vietnam syndrome still stalked American policy. Many in Congress remained sceptical and had to be persuaded that the war would succeed, enjoy broad international backing and not impose massive costs on US taxpayers. Second, it was believed that securing a UN authorisation would strengthen the legitimacy of the intervention. Third, operating alongside coalition partners that fought and, crucially, paid could blunt domestic criticisms that the United States was financing the war alone.

Why did so many states support the war militarily or financially, even when some remained unconvinced that force was necessary? The conflict was widely understood as the founding enforcement action of the post-Cold War order. The end of bipolarity had opened the possibility of a new international system: one that would be multilateral in form and heavily dependent on American power in substance. US leaders framed the crisis not simply as a regional conflict over Kuwait, but as the first major test of what the international order after the Cold War would look like.

Britain and France both saw the war in those terms. Prime Minister John Major presented the conflict as a test of the authority of the United Nations and of the principle that states should not profit from aggression against their neighbours. Britain could not remain passive at such a moment of international reordering. President François Mitterrand reached a similar conclusion. Once diplomacy failed, Paris judged that staying out would weaken both France’s standing and the UN-centred order it claimed to defend.

Germany’s role during the Gulf War was different. Reunified, prosperous and deeply grateful for Washington’s support during reunification, Bonn faced intense American pressure to help finance the war. Baker’s appeal to Chancellor Kohl relied on explicit reciprocity: because the United States had strongly backed German reunification, Germany was expected to demonstrate solidarity with Washington by underwriting a substantial share of the war effort, even if constitutional and political constraints kept German forces out of combat. 

European governments also responded to the 2003 US-led invasion of Iraq in a variety of ways.

In March 2003, a ‘coalition of the willing’ composed of 48 nations offered political, military and/or logistical support for the US intervention to topple Saddam Hussein’s regime. A total of 38 states later sent ground troops. As the war grew more controversial, the administration of George W. Bush’s need for coalition partners increased. Allies could help mute criticism of the war’s alleged illegality and share the costs of occupation and reconstruction.

The UK joined for reasons that remain debated, but the declassified historical record illustrated that, as US resolve hardened, Tony Blair and his inner circle moved from trying to restrain American planning to trying to influence it. Blair hoped British participation would buy leverage in Washington. His government tried to present support for regime change as conditional on Saddam’s further isolation, a continuation of work through the UN, and renewed American commitment to the Middle East peace process. In effect, Blair sought a quid pro quo: British participation in Iraq in exchange for greater US engagement on a broader Middle East settlement.

For many Eastern Europeans, the logic was different. The Iraq crisis came just as some had recently joined NATO and others were about to do so. Their governments wanted to prove that they would be reliable Atlantic allies rather than hesitant consumers of western security. The clearest expression of this logic was the February 2003 Vilnius Group statement by 10 East European countries, in which they declared themselves ready to contribute to an international coalition if Iraq failed to comply with UN demands. This was not only a position on Iraq; it was also a message to Washington.

The United States reinforced these incentives with generous subsidies. Poland received just under $1 billion between 2003 and 2007 for leading and participating in one of Iraq’s multinational divisions. Warsaw also hoped for side benefits, including progress on missile defence and visa-free travel. Other East European states were similarly compensated through deployment support and military aid. Bulgaria obtained an additional political gain when Washington designated it a market-based economy shortly before the invasion and reimbursed Iraqi debt owed to Sofia. Here, too, coalition participation reflected not consensus on the war’s wisdom but calculations about dependence, loyalty and potential rewards.

With war raging once again in the Middle East, Europeans have so far refused to join the United States in its confrontation with Iran. Is the reason simply that they do not share Washington’s threat perception, or that their military and political attention is absorbed by Russia? Those factors matter, but they are not sufficient to explain their reluctance. Europe has faced similar divergences in threat perception and similar strategic preoccupations before, yet these did not prevent participation in earlier US-led wars.

Part of the answer lies in the political atmosphere that has defined transatlantic relations since Trump returned to office. European governments have been humiliated, disparaged and sidelined. In the rhetoric of the new administration, Europe has appeared less as a partner than as an object of contempt: militarily enfeebled, economically marginal, politically decadent and fated for civilisational decline. Trump’s attempt to strong-arm Denmark over Greenland was only the crudest expression of his administration’s new posture towards the continent and its political elites.

Another part of the answer may lie in Trump’s unwillingness to bargain. As earlier cases show, some European contributions were secured through explicit or implicit quid pro quos.

The most important explanation appears to be a structural change in transatlantic relations. In most earlier cases, American power made resistance seem too costly. European governments joined not because they shared Washington’s reading of the threat – often they did not – but because alienating the United States appeared dangerous. In Korea, European participation followed intense US pressure, which was backed up by a relationship of asymmetrical dependence. During Vietnam, several governments feared above all the consequences of undermining their relationship with Washington, a factor that influenced their decisions to assist American’s efforts even as they shied away from joining the military campaign. In the Gulf War, European powers did not want to be excluded from a moment in which the United States seemed to be refashioning the post-Cold War order. In 2003, much of ‘new Europe’ backed the Iraq invasion to prove loyalty to the leader of the free world.

The logic that has previously underpinned European support for American wars may now be weakening. Where many European governments once judged US backing to be indispensable, at almost any political price, they are now willing to risk provoking Washington’s ire. Dependence on the United States – in military and economic affairs – has not disappeared, but a growing number of European capitals have begun to act on what would once have seemed an unthinkable premise: that they must learn to navigate a world in which American power can no longer be treated as the unquestioned centre of international order.

For some European states, embracing the post-American world order comes easier than for others. Spain, for instance, has emerged as one of the most outspoken European critics of the ongoing war with Iran. Its position is underpinned by several structural factors: membership in the European Union, a strong economy that grew by 3.2 per cent in 2025, a comparatively reduced degree of energy vulnerability, given its ability to generate essentially all of its domestic electricity consumption at home, and limited direct exposure to Russian security threats because of geographical distance. The United Kingdom stands in a markedly different position. It remains deeply dependent on the United States, above all in military terms. British defence relies heavily on US high-end capabilities, particularly in the areas of nuclear deterrence, intelligence and aspects of air power. This dependence is also economic: in 2025, the United States was the UK’s largest trading partner, accounting for 17.5 per cent of total British trade. This asymmetry may help explain why the current British government appears to be considering how it can provide some form of support for the United States in the Iran war.

Is it prudent for Europe to embrace a post-American logic and refrain from assisting the United States in its war with Iran? Some would contend that such a course could further hasten an American withdrawal from Europe, as recent remarks by President Trump indicate. Yet this may already be happening; the process of US disengagement from Europe is already in motion regardless of Europe’s choices. Others, by contrast, argue that American decision-making remains erratic rather than settled, and that there is still a chance that the United States, perhaps under a different administration, will come to recognise Europe’s strategic importance and remain engaged in the future. It is difficult to know whether Europe’s resistance now will have a significant impact on US actions in the future. At present, however, there seems no prospect of a meaningful reversal in the position of European governments, notwithstanding the growing intensity of American threats.