The aim of this study is to establish the scale and importance of English intervention during the French Wars of Religion of the late sixteenth century. The author looks at why and how Elizabeth I intervened, and what were the consequences of this intervention. He examines how the 'natural' enemy became an ally and how relations between Elizabeth and three French kings were frequently at the heart of grand strategy. Elizabeth's sword of intervention was double-edged: both benevolent and exploitative. She hoped to aid the Huguenots while recovering England's lost French territories. Later, her intervention became a method of keeping hostilities with Spain away from English shores. Many believed intervention was necessary to protect the continuation of English trade with Europe. England became the 'arsenal' of first Huguenot, then royalist France. The author measures, for the first time, the scale of provision of matériel de guerre. He examines the role of economic and monetary questions and shows how England effectively 'kickstarted' and perpetuated the wars. The emphasis is placed on military history, focusing on the involvement of English armies at Le Havre (1562-63), Rouen (1591), Crozon (1594) and Amiens (1597). Exploiting much previously untouched material from English and French libraries and archives, the author's research reveals the real strategy and tactics of Henri IV, allowing a re-evaluation of this military leader.