In light of the Arab Spring and after days of public quarreling that highlighted the divisions among NATO's members on an agreement to give command of the "no-fly" zone in Libya to the Alliance, it is evident that the U.S. is having problems engaging with its European allies and partners. Why is this happening? Breaking away from the conventional way to study transatlantic relations, Serena Simoni uses a Constructivist theoretical lens to argue that the transatlantic partners' changing identities since the early 1990s have influenced their political interests and, as a consequence, their national security policies. Contemporary divergences are a notable byproduct of these transformations. By focusing on cases of disagreement (i.e., NATO's enlargement, the International Criminal Court, and Debt Relief for Africa), this book shows how since the 1990s, the US has started to see itself as the actor carrying the international defense burden, while the European Union has developed an image of itself as the actor in charge of humanitarian efforts, which generally entails diplomacy rather than military efforts. Contemporary cases of disagreement as the Arab Spring, Libya, and Foreign Assistance in Africa illustrate how redefined national identities continue to alter the course of transatlantic relations. Understanding Transatlantic Relations provides a more accurate examination of the future of transatlantic relations and offers an understanding of those issues that the United States and Europe would consider important enough to justify their cooperation.