The idea for this book first began to take shape in New York towards the end of 1989 when I, like millions across the globe, watched the televised images of the extraordinary political upheavals sweeping across the Soviet Union and Central Europe, including the tearing down of the Berlin Wall and the collapse of the German Democratic Republic. The intellectual climate of those months was quickened with a sense that one was watching the human equivalent of a tectonic shift in the settled political landscape of the late twentieth century. For someone like myself who had grown up in the Irish midlands, and who was keenly aware in 1989 that Northern Ireland was starting into the third decade of a long-running war, the sources of which lay in a partition settlement established in the 1920s, the fall of the Berlin Wall, and the subsequent reunification of Germany, provoked a whole series of questions about nation and state formation, state division and the significance of partition. What are the conditions, I wondered then, that would explain why in some situations partitions collapse or prove reversible, while in others they appear to become permanent, and in others still the issue of division or reunification seems destined to remain a matter of constant contention?