The conventional American adage that “politics ends at the water’s edge” is no longer taken for granted across the democratic world. Political parties and domestic actors actively contest foreign and security policy, undermining pre-existing academic expectations about cross-party consensus as a defining feature of foreign and security policy. Recent scholarship has delved into how partisan contestation influences states’ foreign and security policy behavior (Hofmann and Martill 2021; Haesebrouck et al., 2022), triggers policy change (Chryssogelos, 2020), and affects a country’s standing in international affairs (Myrick, 2021). This shift towards viewing foreign and security policy as contested arenas, akin to other domestic policy fields, is supported by a growing body of empirical evidence (Raunio & Wagner, 2020; Saunders, 2024). However, this perspective underappreciates the possibility that partisan contestation might actually foster cross-party consensus. In other words, scholars might ignore how contestation may itself lead to consensus under certain conditions. Both partisan disagreements and cross-party consensus are recurring phenomena in the political arena, with parties often diverging on the objectives and strategies of foreign and security policy. Nonetheless, such disputes can lead to cross-party consensus, with previous opposing parties abandoning their opposition and joining their rivals to support the same policy orientations or solutions. This dissertation explores the role of reinforcing political pressures to explain how crossparty consensus may emerge out of episodes of partisan contestation over foreign and security policy. It argues that exposure to reinforcing pressures from above, coming from international partners, and from below, originating from opinion leaders and the domestic public, together with horizontal pressure from rival parties, forces a party to reconsider its political objectives in a way that enables cross-party consensus. In explaining how reinforcing pressures lead to cross-party consensus, the dissertation introduces the concept of political entrapment as the mechanism through which consensus is reached. Exposure to reinforcing political pressures, it is said, forces a party into a situation where it must tradeoff between its office-seeking and policy-seeking motivations, prompting intra-party struggles that lead to policy shifts, enabling consensus. This process demonstrates that consensus can emerge through the exertion of political pressure rather than through other causal channels, such as bargaining or persuasion, as some other scholars may argue. Additionally, this dissertation contributes to the debate between culturalists and party politics scholars in international relations (IR) and foreign policy analysis (FPA) concerning the influence of culture and ideology on political motivations and policy outcomes (Hofmann, 2021). Drawing on the latest advancements in strategic culture scholarship (e.g., Bloomfield, 2012), this dissertation argues for a synthesis of party politics and culturalist approaches. Recognizing that strategic cultures can be internally heterogeneous and thus prone to contain subcultures with slightly different beliefs about the use of force helps us bridge what this dissertation suggests is an artificial divide between two bodies of scholarship that provide compatible explanations of political motivations and foreign policy change. Empirically, the dissertation examines the evolution of German foreign and security policy post-reunification, highlighting Germany’s unique historical context and its patterns of fluctuating between contestation and consensus. Through a detailed analysis of three case studies, it tests the theoretical assertions made therein: the NATO intervention in Kosovo (1998-1999), which illustrates how reinforcing pressures can politically entrap parties to facilitate a cross-party consensus; the debate over the US-led invasion of Iraq (2002-2003), a negative case where a consensus was obstructed due to an absence of reinforcing pressures; and the recent controversy over arms deliveries to Ukraine following the 2022 Russian invasion, where the Social Democrats’ policy shift underlines the effectiveness of political entrapment. Employing a longitudinal comparative case study research design and employing process tracing as a within-case method, supplemented by content analysis and archival research, this dissertation offers a nuanced understanding of how consensus over foreign and security policy can emerge out of partisan contestation.