This article considers US-French collaboration on Lebanon, especially between 2004 and 2008. It examines the political background to such collaboration and its manifestations at the United Nations Security Council and in the two powers' relations with Lebanon, Syria, and other regional players. We argue that the changed political landscape in the Middle East following the 2003 US invasion of Iraq (particularly Syrian policy in Lebanon and towards Iraq) as well as developments in the Lebanese theater since the turn of the 21st century prompted such collaboration. After briefly discussing the insights of Realist and Liberal Internationalist theories of international relations, the article concludes that Daniel Deudney's Republican Security Theory offers the most plausible explanation for US-French collaboration on Lebanon. The literature on the international relations of the Middle East has devoted limited attention to how the United States and major European powers (primarily France and Britain, but increasingly the European Union) have worked together or at cross-purposes in pursuit of their interests in the region. Published work tended to emphasize US-European rivalries and competition, highlighting how the United States in the late 1940s and 1950s sought to replace France and Britain as the dominant power in the region.1 Breaking away from the trend that stresses conflict among the major powers, this article analyzes US-French collaboration on Lebanon since 2004. In addition to highlighting the significance of Lebanon to the US-French rapprochement in the aftermath of the Iraq War, this article addresses a number of questions regarding why France and the United States decided to work together in Lebanon and the implications of their close collaboration for the Lebanese internal situation and for US-French relations in the Middle East. We will first consider the ebbs and flows in US-French relations from the aftermath of the 9/11 attacks on the United States until the passage of UN Security Council (UNSC) Resolution 1559 (September 2004) which was, from the outset, a joint USFrench initiative. We will then look at how the assassination of former Lebanese Prime Minister Rafiq Hariri in February 2005 drove France and the United States to intensify their collaboration on Lebanon. The third section examines how the two powers responded to the summer 2006 war between Israel and Hizbullah and subsequent developments in Lebanon that weakened their local partners. Fourth, we shed light on the relative merits of Realist and Liberal Internationalist explanations for the emergence of US-French collaboration. Finally, the conclusion suggests that Daniel Deudney's Republican Security Theory does a better job than Realism and Liberal Internationalism in explaining US-French collaboration in Lebanon.2 US-French relati ons from the attac ks of 9/11 until the passage of UN Security Council Resolution 1559 The United States and France have been significant players on the Middle East scene for many decades. While their long and sustained engagement in the affairs of the region has led to a number of collisions,3 the overall relationship remained sufficiently amicable to permit collaboration whenever the two countries perceived their interests as converging. Thus, in the 1980s, the United States and France collaborated to deny Iran victory in its long war with Iraq; and in 1990, France, despite its initial push for a diplomatic solution, joined the military coalition that the United States formed to drive Iraq out of Kuwait and later took part in the UN-sanctioned surveillance operation that established no-fly zones in northern and southern Iraq.4 The 21st century brought opportunities for both conflict and cooperation. In the wake of the 9/11 attacks, the French government and media expressed solidarity with the United States. France's leading newspaper Le Monde carried the headline: Nous sommes tous Americains! …