How do disagreements within multiparty coalitions affect foreign policy, and how can junior parties exert their influence? These questions are of growing importance as new media increases domestic pressure on politicians to interject in international events, and as foreign affairs become more salient in domestic political contestation. Whereas prior research on foreign policy in multiparty coalitions focuses on the influence of junior parties over cabinet decisions, this article proposes a new theoretical concept of ‘rogue decisions’ to describe a distinct outcome of partisan disagreements. Rogue decisions are autonomous decisions by junior parties impacting foreign affairs, taken without cabinet coordination, that undermine their senior partners’ foreign policies. This resembles the ‘anarchy model’ of foreign policy previously attributed to less institutionalized systems. Having identified the necessary conditions that make rogue decisions possible, and the factors that increase their likelihood, the analysis is applied to cases in Britain and Israel. These represent polar opposite parliamentary systems, with Israel among the most proportional, where rogue decisions may be most expected, and Britain the most majoritarian, where they would be least expected. The identification of rogue decisions in contrasting parliamentary democracies challenges the assumption that cabinet is where foreign policy disagreements are managed, according to established decision-making rules. The article therefore prompts new thinking about the potential for junior parties to disrupt the foreign policy agendas of their senior partners, and challenges in new ways the assumption that states act as coherent units.