For centuries the defence of the realm against foreign attack had been a major consideration when British statesmen dealt with Ireland. It drove the conquest and plantation of Ireland in the sixteenth and seventeenth centuries, the passing of the Act of Union in 1801, and opposition to Irish Home Rule in the later nineteenth and early twentieth centuries. Although it fell into abeyance during the 1920s and early 1930s, the deteriorating international situation made it of dominant importance after 1936. In that year Adolf Hitler remilitarised the Rhineland and accelerated a massive armaments programme, while Mussolini's Italy and an expansionist Japan threatened Britain's position in the Mediterranean and the Far East. In response to these challenges the British government belatedly rearmed, sought alliances with friendly states and attempted to avoid conflict by acceding to the demands of the fascist dictators. The policy of appeasement was taken to its furthest extent by Neville Chamberlain, who became Prime Minister on 28 May 1937. Yet he could do little to arrest the world's slide towards armed conflict. Because of its position as England's ‘back door’, Ireland would have a vital strategic role in this struggle. Consequently, from 1936 defence became the most powerful factor in Anglo-Irish relations, trumping the long-held principles and preconceptions that shaped British policy towards Ireland.

The menacing international situation and the increasing importance of defence made intelligence on Irish security threats more vital than ever. The fundamental question for the British government was to what extent southern Ireland would assist in the defence of the British empire if war came. Would it be a hostile presence on Britain's flank? Would it participate in the war along-side Britain? Or would it adopt a stance somewhere in between – neutral but co-operative? An assessment of the likelihood of these outcomes was necessary to inform British policy-makers as they sought to negotiate a settlement with de Valera in 1938. The British government also faced real security threats during this period, in the form of German propaganda and espionage activities in Ireland, and an IRA bombing campaign on English soil.