The specter of an increasingly uneven distribution of power between the Executive Branch and Congress looms large in scholarship on the separation of powers in American government. This is particularly evident in the design and exercise of foreign policy. While the presidents’ use of legislative powers like the veto has been studied extensively, I argue that preemptive presidential interventions during the process of lawmaking are a subtler and less politically costly tool that is more important than scholars realized. This project focuses on American foreign policy lawmaking and examines the extent to which it is characterized by presidential assertiveness communicated to Congress through a hitherto understudies tool of executive intervention. Specifically, I seek to understand how contemporary presidents use Statements of Administration Policy (SAPs), a relatively new class of executive communications designed to preemptively intervene in key legislation as it is drafted in Congress. Delivered at a crucial intervention point along the legislative process, these communications provide a key opportunity for presidents to challenge specific provisions. Embedded in the broader discourse on struggling congressional checks on the executive, this project fills a gap in the understanding of the dynamics that shape the balance of power between Congress and the president. I find that interbranch confrontations increased significantly after the critical juncture of 9/11 and that Congress gradually expanded the use of its authorization leverage over time. In addition, two contextual predictors of increasing presidential assertiveness in preemptive messages to Congress stand out: divided government and presidential election years. My results indicate that presidents still cannot achieve many of their foreign policy goal without negotiating with Congress within contested bargaining procedures. Overall, my dissertation makes three principal contributions to the ongoing research on domestic underpinnings of US foreign policy: First, my findings quality the image of unchecked presidential discretion and highlight the continuous relevance of interbranch contestation in foreign policy. Second, I show that the mechanisms of change as laid out by historical institutionalism are useful for explaining long-term shifts in interbranch dynamics. Third, I introduce a novel assertiveness-score based on a relatively new source of presidential position language, which can be adapted for other research purposes.