From Diverging Values to Uncertain Transatlantic Relations: Conceptualising Relational Uncertainty in EU–US Security Relations

Transatlantic relations have always been dynamic, shaped by geopolitics and domestic politics. Against the backdrop of the EU's ambition to achieve strategic autonomy, the second Trump administration has reignited a key question: are current developments a rupture, or merely an acceleration of long-term trends? One perspective, put forward by EU policymakers, is that shared meanings and norms have eroded to an unprecedented extent, rendering cooperation along established lines impossible. A second interpretation emphasises that today’s tensions, e.g. Washington’s pivot to Asia or pressure on Europe to increase defence spending, reflect structural shifts that predicted Trump, suggesting continuity. Given that the EU still depends on the US to a large extent for its security and defence, it is essential to assess the evolving nature of the transatlantic relationship, as different perceptions also can contribute to diverging views on strategic autonomy. This paper argues that the changes under 'Trump 2.0' can be better understood when viewed through the lens of the uncertainty surrounding US foreign policy towards Europe. Although uncertainty is frequently invoked in the study of EU–US relations, often in the context of the need for strategic autonomy, its analytical potential remains underdeveloped. In an effort to address this shortcoming in the literature on transatlantic relations, this paper proposes an actor-centred, relational framework for analysing uncertainty within the transatlantic context, focusing on its impact on EU security and defence policies. Existing theoretical scholarship highlights that uncertainty is multifaceted, extending beyond missing information to include subjective interpretations, cognitive limits, and socially constructed frames. Although structure can generate uncertainty, human agency plays a pivotal role in its production, perception, and response. Uncertainty can function as both a constraint and an opportunity, enabling actors to gain influence, create opportunities for policy change, or reinforce existing preferences. However, current approaches locate agency in the production of uncertainty either in the withholding of information by actor A, or in the subjective processes of actor B such as interpretation, misperception, or bounded rationality, implicitly assuming a one-directional relationship. This paper instead argues that uncertainty can also arise bidirectionally from the relationship itself, when the values, norms, and beliefs of one actor are unknown, or only partially align with those of another actor. In such cases, a lack of shared values, norms, and beliefs, or insufficient information about them, undermines actors’ ability to predict each other’s future behaviour. This should not be confused with the constructivist claim that divergent identities cause actors to interpret information differently, since this requires information to be present in the first place. The argument here is that unknown values or value divergence weaken actors’ ability to predict each other’s future behaviour, leading to uncertainty. While generating mainly uncertainty about the future, it influences policy decisions today. Although this paper is theoretical in nature, it uses empirical examples to support its claims. This framework can then be used to examine how uncertainty influences European responses, decision-making and preference formation in security and defence under Trump 2.0.