NATO recently agreed to a historic 5% defence spending target. But Osman Sabri Kiratli presents new experimental evidence revealing that what Americans truly care about goes far beyond the numbers. In fact, democratic allies may have more leeway than they realise
For years, NATO defence spending has dominated headlines. At a February 2025 rally, Trump even threatened to encourage Russian aggression against underspending members. Such threats have partially paid off: At the 2025 NATO summit in The Hague, alliance members agreed to an unprecedented commitment: 5% of GDP on defence and security by 2035. This more than doubles the previous 2% target and represents a $1.9 trillion annual increase across the alliance. European leaders made this historic concession largely to satisfy American demands for greater burden sharing.
But will it be enough? And, more fundamentally, what happens when allies act in ways that fail to satisfy American leaders, regardless of their spending levels?
Alliances inevitably involve policy disagreements. The credibility of any American president’s threats against uncooperative allies depends fundamentally on domestic support. When such pressures lack popular legitimacy, target states can anticipate limited follow-through and resist.
Conversely, measures with robust public backing carry credible diplomatic leverage.
This matters for European policymakers. Understanding what American voters will or will not tolerate helps predict which alliance frictions might escalate and which remain manageable. My survey experiment of over 1,500 Americans explores where these lines are drawn.
When policy frictions arise between the US and its allies, American voters strongly reject inaction. I tested public reactions to five potential US responses against allies: public denouncement, economic sanctions, reduced military aid, alliance termination, or maintaining current policies.
The results are stark. Economic sanctions and cutting military support receive overwhelming approval; significantly more than rhetorical criticism. Americans favour tangible coercion over diplomatic statements. This suggests that US presidents may face intense domestic pressure to take visible action against partners that do not seem to reinforce American 'leadership'.
When policy frictions arise between the US and its allies, Americans favour tangible coercion over diplomatic statements
Interestingly, breaking alliances ranks as the least popular option. Americans want enforcement, not abandonment. They favour measures preserving strategic relationships while penalising problematic behaviour. This suggests certain limits to Trump’s more extreme threats, even if his base supports pressure tactics.
Results also show that when assessing ally behaviour, Americans care far less about underspending than policymakers assume. When presented with different types of allied non-cooperation — underspending on defence, refusing troop deployments, purchasing Russian or Chinese weapons, blocking new alliance members, or breaking US sanctions — purchasing weapons from rivals provoked the strongest negative response, followed by refusing troop deployments. Financial underspending generated the mildest reaction.
This creates both risk and opportunity. Germany’s path to 5% matters less politically than Turkey’s purchase of Russian S-400 missiles that was followed by Countering America's Adversaries Through Sanctions Act sanctions since 2020. Poland’s troop contributions to joint operations count more than Italy’s defence budget. European leaders fixating on spending percentages may miss what truly concerns American voters: strategic misalignment on operational cooperation.
A crucial finding for European allies is that regime type matters greatly. Americans show marked reluctance to endorse punitive measures against fellow democracies, even when they do not cooperate enough.
Support for economic sanctions drops by nearly a third when targeting democratic rather than authoritarian allies. Approval for breaking alliances falls even more steeply. Shared democratic values do not just reduce conflict between democracies, they also constrain how democracies pressure each other. Surprisingly, neither military capabilities nor formal treaty status significantly affects American attitudes.
While American voters support pressuring allies in principle, they instinctively seek to preserve democratic partnerships
This finding should reassure European capitals racing toward the 5% target. While American voters support pressuring allies in principle, they instinctively seek to preserve democratic partnerships. Germany, France, and Spain all benefit from a democratic premium that limits acceptable coercion, regardless of whether they hit their spending targets on schedule. Despite Trump’s rhetoric and appreciation for strong leaders, Hungary, Turkey, and other partially democratic partners face higher constraints.
Only nine countries globally spent 5% or more in 2023, most of which were either authoritarian petrostates or were at war. Getting there requires massive fiscal sacrifices; Spain’s prime minister called it 'unreasonable' and 'counterproductive'.
Yet, this enormous commitment may not fully satisfy American voters, or President Trump, if transatlantic misalignments persist.
European policymakers should draw three lessons. First, they must prioritise substantive cooperation over symbolic targets. Closer integration of transatlantic defence industrial bases, keeping the rising operational tempo in joint operations driven by heightened security threats, and maintaining strategic alignment all matter more politically than marginal spending increases.
Second, Europe must take advantage of the democratic premium. The transatlantic relationship benefits from deep bonds constraining even populist pressures. American voters instinctively protect partnerships with fellow democracies. This is not charity. It reflects shared values and strategic thinking that transcends any single presidency.
Third, leaders must distinguish between rhetoric and reality. Trump can threaten aggressive measures to satisfy domestic audiences even if democratic constraints limit actual implementation. The 5% agreement demonstrates European willingness to accommodate demands, potentially reducing pressure for more extreme measures.
Strong American support for corrective measures does not make them wise policy. Sanctions and aid cuts often trigger resistance rather than compliance, especially when generating rally-round-the-flag effects. European leaders should recognise that American pressure campaigns may backfire domestically, strengthening nationalist politicians who resist external demands.
Americans want allied states to contribute fairly, but they value democratic partnerships and strategic cooperation more than whether a state has met NATO's defence spending target
More fundamentally, coercion risks long-term relationship damage. Today’s pressure secures concessions but undermines trust for future cooperation. American and European leaders must weigh immediate political benefits against strategic costs.
As NATO races toward 5%, understanding the domestic politics on both sides, not just elite preferences but public attitudes, remains essential. Americans want allies to contribute fairly, but they value democratic partnerships and strategic cooperation over budget percentages. European leaders who grasp this distinction can navigate Trump’s second term while preserving the alliance’s essential character — and without drowning in petty politics.
