In this, the first fully documented study of British and Irish popular reactions to the outbreak of the First World War, Catriona Pennell explores UK public opinion of the time, successfully challenging post-war constructions of 'war enthusiasm' in the British case, and disengagement in the Irish. Drawing from a vast array of contemporary diaries, letters, journals, and newspaper accounts from across the UK, A Kingdom United explores what people felt, and how they acted, in response to an unanticipated and unprecedented crisis. It is a history of both ordinary people and elite figures in extraordinary times. Pennell demonstrates that describing the reactions of over 40 million British and Irish people to the outbreak of war as either enthusiastic in the British case, or disengaged in the Irish, is over-simplified and inadequate. Emotional reactions to the war were ambiguous and complex, and changed over time. By the end of 1914 the populations of England, Scotland, Wales, and Ireland had largely embraced the war, but the war had also embraced them and showed no signs of relinquishing its grip. The five months from August to December 1914 set the shape of much that was to follow. A Kingdom United describes and explains the twenty-week formative process in order to deepen our understanding of British and Irish entry into war.