During the last year of the First World War, on the night of 17/18 May, over 70 leading members of Sinn Fein were arrested under the terms of the Defence of the Realm Act. The arrests had been made following the discovery of a supposed plot on the part of Sinn Féin to help Germany enter Ireland. This “German Plot” is generally believed to have been used as an excuse to intern the leaders of Sinn Féin, who were seen by the British Government as being the strongest force behind opposition in Ireland to the proposed introduction of conscription there. Most of the Irish prisoners were brought to England on 18 May and sent to the prisons at Birmingham, Durham, Holloway, Lincoln, Reading, and Gloucester.

Republicans were tipped off about the impending arrests, allowing some to escape capture while others chose to be taken in order to secure a propaganda victory. The internment was counterproductive for the British, imprisoning the more accommodating Sinn Féin leadership while failing to capture members of the Irish Republican Brotherhood more committed to physical force republicanism. This allowed Michael Collins to consolidate his control of the organisation and put it on a more focused military footing.

Even at the time, the proposition that the Sinn Féin leadership were directly planning with the German authorities to open another military front in Ireland was largely seen as spurious. Irish nationalists generally view the “German Plot” not as an intelligence failure but as a black propaganda project to discredit the Sinn Féin movement, particularly to an uninformed public in the United States. It is still a matter of study and conjecture what impact it had on US foreign policy regarding the 1919 bid for international recognition of the Irish Republic.