The First World War set in train the development of ideas and traditions that had profound implications for nations and for national identity. Whilst the British Empire grew in size at war’s end, revolution and war beset the United Kingdom, the very heart of that empire, resulting in the establishment of what ultimately became the Republic of Ireland. This was a violent rejection of Britishness. Elsewhere within the empire, a shared British identity was simultaneously reaffirmed and undermined by the war. One of the ways in which this manifested itself was through commemoration. This chapter uses the early years of the commemoration of the Gallipoli campaign as a means to observe both continuities within Britishness and the seeds of its decline.