This book examines the militant Irish republican movement in the United States from the final months of the Irish Civil War through to the Second World War. The narrative, crafted to appeal to both an academic and general audience, carefully and creatively intertwines the personalities, events and policies that shaped the militant republican movement in the US during this period and shows the evolution of its deep transnational nature. Most importantly, through a bottom-up historical analysis that incorporates an examination of more than eighty archival collections in the US, Ireland and Britain, the work presents for the first time an account of the anti-Treaty IRA veterans who arrived in the US after the Irish Civil War. Upon their settlement in Irish-American communities, these republicans directly influenced and guided the US-based militant republican organisation, Clan na Gael, transformed the overall dynamics of militant Irish republicanism in America and provided leadership and co-ordination for an IRA bombing campaign. The inclusion of these IRA veterans in the narrative creates a fresh and revised interpretation of the militant Irish republican activism that occurred in the US in the immediate decades after the Irish revolutionary period.