This thesis examines the creation of several ex-service organisations during and shortly after the First World War, the campaigns that preceded their formation, and the processes that led to the amalgamation of four of them into the British Legion in July 1921. Employing previously unutilised sources, it offers a new analysis of activities during the first two years of the war, a period that has not been addressed, to any significant extent, by previous scholarship in this context. Those activities were important as they led to the founding of the first two groups, the National Association of Discharged Sailors and Soldiers ('the Association'), and the National Federation of Discharged and Demobilised Sailors and Soldiers ('the Federation'). These new bodies were quite different from traditional service charities, in that they were democratic membership organisations, run by the members for mutual support, and to press for better government treatment of them and their dependents. This thesis also re-examines the period from autumn 1916 to July 1921, which saw the growth of those two organisations, and the founding of two rival groups in particular, the Comrades of the Great War ('the Comrades'), and the Officers' Association. The whole process is also placed in the wider historical context of the development of what in recent years has become known as 'the Military Covenant', that is to say, the special obligation that has been observed to exist from the time of ancient Athens to the present day, for the state to support and care for those who take up arms on its behalf, and their dependents. While previous scholars have addressed aspects of the development of these groups, and subsequent ones, from late 1916 to July 1921, there is much new information and analysis here, with more complete coverage of regional developments across the whole United Kingdom, and a number of their assumptions and conclusions are challenged. The major sources employed in this study for the whole period 1914-21 are trades council archives, the papers of James Myles Hogge, Sir Ian Hamilton, Wilfrid Ashley (Baron Mount Temple), Lord Davies of Llandinam, Viscount (Waldorf) Astor, and the records of the Comrades in Scotland and Northern Ireland, all of which have received little attention previously, in relation to this subject. The approach is not that of a military historian, concerned with the prosecution of the war, but rather that of the social historian, examining the conditions of the millions of people who became service families for a few years, or veterans when their service was over, of the claims their organisations made on the British state, and the responses of the Government, Army, and Admiralty, to those demands. As such, it aims to contribute to the study of 'war and social change', as developed by historians such as, in particular, Arthur Marwick. The main new findings are as follows: • Trades councils' campaigns on behalf of service families, and ex-service men and women, from August 1914 to September 1916, were the foundation for their creation of the National Association. • James Hogge's concurrent but independent campaign for the same groups, in Parliament, in the Press, and through individual casework, from August 1914 to spring 1917, was similarly the foundation for his creation of the National Federation. • The two findings above, regarding trades councils, and James Hogge, are in contrast to previous studies that focus on the period after autumn 1916, thus overlooking the important roles their activities played in the founding of the first two organisations. • The fundamental demand of the trades councils, Hogge, the Association, and the Federation, was that these people should be supported by the British state as of right, not by Royal Bounty, or by charity. • Lord Derby, the War Secretary in 1917, is proven, with the help of newly discovered archive papers, to have been a prime mover in the creation of the Comrades, with the backing of the War Cabinet, and he largely succeeded in hiding his involvement from the public. • Hamilton's scheme for an Empire Services League did not fail, as asserted by Graham Wootton, because of opposition from the Association and the Federation, but because Viscount Peel and Wilfrid Ashley (a leading member of the Comrades) conspired to remove essential democratic principles from the proposed constitution, thus rendering it unacceptable to those two organisations. • The Federation did not abandon electoral politics after its disappointing results in the General Election of December 1918, as asserted by Stephen Ward, but continued its active involvement until the middle of 1920, when it dropped this, in order to facilitate amalgamation talks. • The creation of the Welsh Legion of Ex-Service Men in early 1920, not previously recognised, was an important early demonstration of the benefits of amalgamation, and a significant early use in the UK of the name 'Legion', as a collective term for ex-service men and women. • The failure of the Federation's long-awaited deputation to Prime Minister Lloyd George, in February 1920, along with plummeting membership and financial reserves during that year, and the earlier launch of Haig's Officers' Association, were major drivers towards the eventual amalgamation of the Association, the Federation, the Comrades, and the Officers' Association, as the British Legion.