Resistance in Europe during the Second World War has long suffered from being studied through a predominantly national lens. The weight of postwar narratives of national reconstruction, the logics of funding bodies and of archives, as well as the wish of many historians to locate resistance within national triptychs of defeat, resistance, and liberation, have all deflected attention from the evident reality that resistance was a phenomenon that spilled beyond state borders in its structures and operations, its personnel, and its mentalities. Indeed, much resistance occurred in spaces outside the nation: within local communities and specific social milieux, and among those whose loyalties and experiences transcended state boundaries. It is this last group—the transnational resistance of those who by choice or circumstance found themselves outside the nation—who are the focus of Robert Gildea and Ismee Tames’s Fighters Across Frontiers: Transnational Resistance in Europe, 1936–48. Written collectively by a team of twenty-three historians of fourteen nationalities, it is the first Europe-wide attempt to explore the resistance against fascism, the Third Reich, and its allies by foreign fighters, political exiles, economic migrants, refugees from persecution, and the agents of external powers. This transnational resistance was almost infinitely diverse, and the editors very wisely eschew any attempt to impose a definition on their subject matter, preferring to allow the wealth of primary material to speak for itself. The series of loosely chronological chapters trace the emergence of the multiple forms and spaces of transnational resistance from the International Brigades in the Spanish Civil War to the military campaigns of the early war years, the camps of political detainees and POWs, the escape lines created for downed airmen and Jewish civilians, and the armed resistance movements that emerged in German-occupied Europe during the latter war years. Transnational resistance flourished especially in the spaces in between—among the partisan groups that operated in the free zones of the western Balkans, and of Italy after the collapse of Fascist rule, or behind the fluid battle lines of the eastern front. These improvised military units received material and organizational support from the Soviet, British, and American forces. But the primary driving forces behind their development was the social chaos generated by the escalation of the war, the policies of repression imposed by the Nazi rulers and their local allies, and the effective collapse of government in large areas of German-occupied Europe. As national structures disappeared, they created opportunities for new armed groups, often highly local and personalized in character, that operated outside the boundaries of state authority. These drew on the large numbers of exiles and migrants displaced over the previous years by persecution and war, for whom resistance was the expression of their political and ideological beliefs as well as a response to their marginal and illegal status. But their ranks were greatly swelled during the final years of the war by former prisoners of war, Jewish refugees, and labor conscripts, as well as by the many civilians obliged to flee their homes to escape ethnic and military conflict. These formed highly heterogenous units, where the labels of national identity were more symbolic than real. Spanish Republicans featured prominently among the Free French units who marched into Paris with the Allied troops; former Italian soldiers fought in the partisan units that wrested Belgrade from German control; while the Slovak National Uprising in 1944 was carried out by a heterogenous coalition of local fighters, Soviet partisans, and British and American agents, supplemented by diverse groups of escaped POWs and displaced conscript workers.