On 1 September 1939 Hitler's troops invaded Poland. Two days later Britain and France declared war on Germany. The deadliest conflict in human history had begun. Éamon de Valera immediately declared that the Irish state, unlike the other Dominions, would remain neutral. Although hardly unexpected, this was the outcome that had haunted British military theorists ever since the Anglo-Irish Treaty was signed in 1921. Irish neutrality held many strategic dangers for the British war effort. First, Ireland presented opportunities for German covert operations against the United Kingdom: its waters and coasts, stretched across vital British trade routes, could be used for the clandestine shelter and supply of U-boats; it was an ideal base for espionage and sabotage; it was fertile ground for German propaganda and agitation. Second, there was a risk that Germany would seize Irish territory, using it as a base for attacks on British cities and shipping, or as a ‘back door’ for invasion of the United Kingdom. The third threat consisted of what Irish neutrality denied to Britain – the use of Irish naval and air bases by British forces. Exclusion from southern Ireland would greatly handicap Britain's ability to protect its Atlantic trade from German attack in the first two years of the war.

These three factors pulled British policy in different directions. Preventing German covert operations and deterring a German invasion required the goodwill and co-operation of the Irish government; on the other hand, access to Irish ports and air bases could only be achieved through pressure on the Irish state. British policy towards Ireland veered between accommodation and confrontation, depending on the fluctuating perceptions of these strategic dangers. Aware that British attitudes were finely poised, Éamon de Valera was prepared to make concessions to ensure that the balance was tipped in his favour. Neutral Ireland showed a ‘certain consideration’ for British needs. The Irish provided Britain with operational intelligence (such as sightings of enemy U-boats and meteorological reports), assisted the repatriation of British airmen who landed on Irish soil, and cracked down on Axis espionage, sabotage and propaganda activities. De Valera summed up this attitude in September 1939, when he assured London that he did ‘not want Irish freedom to become a source of British insecurity’.