Department chairs play a key role in efforts to diversify higher education, particularly in fields like computer science that face long-standing gender and racial/ethnic gaps. This study considers the role of computer science department chairs in guiding broadening participation efforts and how they make sense of external dynamics that influence their efforts. Objective: This study seeks to better understand the external dynamics that shaped diversity efforts within their departments and how chairs positioned themselves in relation to those dynamics. Method: Using the Inclusive Excellence Model as a conceptual framework this longitudinal qualitative study draws on 29 interviews conducted over two years with 18 computer science department chairs from 15 research universities across the United States. Findings: Chairs described actively grappling with and leveraging external dynamics across three dimensions: "systemic", "political", and "bureaucratic/structural". While existing literature on department chairs often frame external dynamics as unidirectionally impacting departmental efforts, we found that chairs saw their role as agentive. Implications: As efforts to broaden participation in computing (BPC) continue to expand, we argue that chairs must recognize and harness the complex and interconnected external forces that influence their work in order to fully actualize their role as transformative leaders.