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. The military leadership of Matilda of Canossa adopts an interdisciplinary perspective towards the abundant and diverse sources for her life, reading the narrative sources against the letters, polemics, diplomas and canonical collections. It combines these to reconstruct Matilda's campaigns in painstaking detail, and reconsiders the limits of medieval women's military agency in light of her demonstrable successes. Both scholarly and accessible, all Latin quotations in the book are translated and the introduction provides a primer on the 'Investiture Contest'. This work will be of greatest value to specialists in medieval gender, military and church history.