There is good reason to include Thomas Becket in discussions of later twelfthcentury military history. The implication of Henry II in the murder of Archbishop Becket in 1170 forced the king to enter into protracted negotiations with the papacy of Alexander III, which seemed inclined towards placing England under censure and perhaps even excommunicating the monarch himself. Two distinct yet related events resulted from this lengthy dispute with the Church, both having military implications. The first was Henry's submission to the will of Alexander in 1172, as the dialogue between papal legates and the king's representatives ended with the signing of the Compromise of Avranches. In his acquiescence, Henry agreed to provide money and manpower for the preservation of the Holy Land. The military impact of Henry's contributions to such crusading efforts is questionable, as is the effect his central funding mechanism, the Saladin Tithe, had upon the English economy. More concrete was the second repercussion of Becket's murder, the rebellions of 1173 and 1174. Various enemies, including the king's own sons and his more traditional opponents Philip of Flanders and Louis VII of France, strengthened by Henry's political weakness, took up arms against the humbled monarch. Avranches and the rebellions were both posthumous retaliations on Becket's part, in a poetic sense, yet there is more to the military side of the Becket story than just the aftermath of his murder.

In the years before Becket became archbishop he was involved in a number of military matters during his days as royal chancellor (1155–1162). On occasion, he even took the field armed for combat and was, if the contemporary accounts are to be believed, wildly successful in his pursuits, by seizing fortified towns and castles, leading cavalry in and out of battle, and engaging in valorous individual combat. The legacy of Thomas the Warrior is, perhaps, not as well remembered as the story of his martyrdom, but his personal involvement in warfare deserves study beyond a casual reading of his activities in the contemporary records. If, as W.L. Warren has argued, Becket's experiences on the battlefield in large part dictated his behavior as archbishop, this military aspect of his life becomes crucial to our understanding of the Becket Affair.