Why members of parliament (MPs) defect from the party line is a frequently researched topic in comparative political science. Yet, the question has an application in international relations and it has implications for the foreign policy decision-making processes of democratically elected governments. This paper suggests that a key determinant, beyond the institutional and individual factors leading to defection, lies on the party level. Specifically, I argue that heterogeneity in how 'dovish' a party has positioned itself will affect the party cohesion in votes over military deployment. I leverage temporal variation in party manifesto programs, to assess the preference heterogeneity over 'dovish' preferences. I assess my hypotheses against parties' voting records in military deployment votes. I rely on fixed-effects models and include the mission, the cabinet, the country, and both the country and year to assess the robustness of my results. I show that conditional on whether a party is in the government, heterogeneity in preferences can have a significant impact on the number of defections from the party line. In a subsequent step, I assess the impact of party preference heterogeneity in coalition governments and argue that a lack of overlap in party preferences can encourage individual MPs to signal discontentment with the coalition partner, leading to a greater number of defections. This paper highlights the role of intra- and inter-party preference heterogeneity in foreign policy decision-making. The results have both empirical and theoretical implications for studies relying on the two-level game to explain international cooperation.