The United States’ invasion of Grenada in 1983 represented the lowest point in Margaret Thatcher and Ronald Reagan’s relationship, with Thatcher incensed at what she perceived to be her ally’s misuse of military force. However, in April 1986, Thatcher gave permission for the United States to use British-based aircraft for air strikes against the Gaddafi regime in Libya, a mission as tenuously grounded in international law as Grenada. How do we explain Thatcher’s apparent change in approach to foreign policy, now placing strategic interests above her previous deference to international law, and what does this tell us about the role experience plays in a leader’s foreign policy decisions? Drawing on insights from the ongoing behavioural revolution in International Relations, this paper argues that the experience Thatcher gained during the Grenada episode led to her support for US strikes against Libya. A process tracing approach using documents from the UK National Archives, as well as biographies and memoirs, tests this individual-level hypothesis against a rival structuralist explanation. This research shows how experience gained in office can influence a leader’s future foreign policy decision-making and demonstrates the utility of process tracing methods for investigations into the role of experience in international politics.