The contribution of the Indian Army to the First World War has been, until recently, at best forgotten, and at worst ignored. This is especially so of those military units provided by the semi-autonomous Indian Princely States whose contribution, even in the Official Histories, barely merits a mention. This book corrects that imbalance. Drawing on a wide range of regimental and Army Headquarters war diaries, few of which have been cited in other publications, as well as political records held exclusively at the National Archive of India, the book explores why the State rulers volunteered their services and those of their armies. It looks at the deployment of those armies in Europe, Egypt and Palestine, Mesopotamia, East Africa, the Dardanelles and within British India. It draws attention to problems encountered in their deployment, but also highlights successes such as the capture of Haifa and Aleppo by State cavalry units, and the outstanding role of some State infantry units in East Africa. One appendix records over 1,600 State soldiers who died – killed in action, died of wounds or as prisoners of war, or died of disease, while other appendices record battle honours and rewards for gallantry or meritorious service. Problems identified during the war were addressed in its aftermath and the book concludes by looking at the recommendations which led to the transformation of the Imperial Service Troops scheme into the Indian State Forces scheme.