This book sets out to explore the contribution of the Society of Jesus to the dissemination of knowledge about military architecture in the Baroque age. It shows how the Jesuits developed a militant form of religious expression targeted at protestants and infidels and how many Jesuit mathematicians assisted Catholic leaders by using the mathematical faculties attached to many colleges and seminaries for nobles to provide classroom and private teaching, publications and even consultancies concerning fortification theory, then considered to form part of the 'mathematical disciplines'. This book reveals that the involvement of many Jesuits in 'de re militari' was widespread, sometimes leading to criticism, internal controversies and crises of conscience. The interest of Jesuits in fortifications was discontinued after the 1773-1814 suppression period.