This book traces the development of British military intelligence from the disasters in the Crimea and through to World War I. On the Western Front, an Intelligence Corps supported the British Expeditionary Force and provided counter intelligence and protective security in rear areas to the Channel ports. Aircraft were first used for Photographic Intelligence. Commanders facing long casualty lists learned that intelligence is more effective than raw courage. Wireless and mail intercepts grew in importance, and a department collected information from prison camps. The East African Intelligence Department recruited big game hunters and Africans. While intelligence failures at Gallipoli still resonated, MI in the Middle East adapted to fighting that was rarely positional. On the Home Front, counter-intelligence minimized the risk of German espionage, sabotage, and subversion and tackled terrorism as the IRA fought for independence. From this moment on, Britain never lost the lead in MI, up to and beyond WWII.