In the summer of 1948 the Taoiseach, John Costello, announced to the world press – without consulting his cabinet – that his government intended to ‘ditch’ Eire’s last remaining constitutional link with the British Commonwealth. Why? Was it a calculated political move or an impulsive outburst spurred on by newspaper comments and the ‘Roaring Meg’ incident in Canada? Newly available research material from American, British, Canadian and Irish archives provides answers to these controversial questions. The formulation of the British government’s response, which took the form of the ‘guarantee’ on Partition to the Parliament of Northern Ireland, is set within the context of the formation of NATO and Eire’s subsequent ‘refusal’ to join the alliance. The book shows how the perennial problem of Partition permeated and debilitated the Coalition government itself and in its diplomatic discussions with Britain, the Commonwealth, America and the NATO powers. As background the book reviews Anglo-Irish constitutional developments from 1932 until 1949, particularly the abdication of King Edward VIII and the introduction of the External Relations Act 1936.