Hertfordshire occupies a dominant position on the northern approaches to London and was traversed by three major Roman roads. It was the location of an oppidum (large fortified settlement) of the Catuvellauni tribe and the important Roman town of Verulamium, now St Albans, which were superseded by the Norman castles of Hertford, Berkhamsted and Waytemore (Bishop’s Stortford). During the Middle Ages, the county was heavily involved in the Anarchy, the Barons’ Wars and the Peasants’ Revolt, and three significant battles of the Wars of the Roses. In the Victorian period, the threat of French invasion precipitated the establishment of the Hertfordshire Rifle Volunteers, a formal incarnation of a county militia with roots in the English Civil War. In both world wars, Hertfordshire was heavily involved in training and munitions production; the de Havilland Mosquito was developed in Hatfield, heavy bombers were built in Radlett and Leavesden, and the SOE ran a secret training facility in the county. In the post-war era, production of the UK’s nuclear deterrent has been centred on Hatfield and Stevenage. Defending Hertfordshire sets all these topics in their historical context, providing a comprehensive and thoroughly researched guide to county’s fascinating military heritage.