This is the first account in English of the entire, 40-year military career of one of the most powerful women of the Middle Ages. Challenging the boundaries between military and gender history, it explains how one famous noblewoman rose to the defence of the reforming papacy, defeated the Holy Roman Emperor and turned the tide of the first great war between Church and State. Despite the general neglect of Matilda in the English-speaking world, sources relating to her life are both abundant and diverse. She issued over 130 surviving diplomas, patronised an influential circle of scholars, corresponded frequently with several popes (including Gregory VII and Urban II), and commissioned the monk Donizo to write her biography in epic Latin verse. Her campaigns also provoked a storm of controversy in the pamphlet literature of the 'Investiture Contest'. The military leadership of Matilda of Canossa adopts an interdisciplinary perspective towards this material, reading the narrative sources against the letters, polemics, diplomas and canonical collections, and combining them all to reconstruct Matilda's campaigns in painstaking detail. The book concludes by reconsidering the limits of medieval women's military agency in light of the countess's demonstrable successes on the battlefield and in the war of propaganda. With all Latin quotations translated, this accessible work will be of greatest value to specialists in medieval gender, military and/or church history, particularly those with an interest in the 'Investiture Contest'.