Warrens lifework on Henry I has changed perceptions of that king forever. Long remembered for his unrestrained sexuality and his cruelty, Henry will now be understood (despite his undeniable faults) chiefly as a great peacemaker on both sides of the Channel. In some thirty articles, Warren proved over and again that Henrys administrative sophistication was unmatched in early-twelfth-century Europe.

Warren established the now standard view that Henry was one of the inventors of administrative kingship: government through standard bureaucratic forms, professional administrators, and a series of institutions linking localities to the center. Even more important, Warren bridged the conquest, both chronologically and geographically. He became internationally famous for studies that emphasized the interrelationship of England and Normandy, so helping create the field of Anglo-Norman history, now established and recognized on every continent save (presumably) Antarctica.

Before devoting himself to Henry I, Warren wrote extensively on Anglo-Saxon and Anglo-Norman military institutions. His Anglo-Saxon Military Institutions on the Eve of the Norman Conquest (Oxford, Clarendon Press, 1962), which won the Triennial Book Prize of the Conference on British Studies, and The Military Organization of Norman England (Oxford, Clarendon, 1965) established Warren as a young and brilliant student of both sides of the divide of the conquest in England. His choice of subjects was a courageous one. English historians had been debating the nature of military, governmental, and administrative institutions for a century, and by 1960 they had solved them to everyones satisfaction. Warrens first book, on the fyrd, was a brave piece of scholarship by an as-yet-unknown American, challenging the work of decades of British scholars. Warrens entry into the debate was not welcomed by all, but eventually the good sense of his arguments and the clarity of his prose won him and his theories admirers and then friends.

Warren was also a splendid generalist. His textbooks, The Making of England, now in its seventh edition, and Medieval Europe, just issued in its eighth edition, are the most frequently used undergraduate textbooks in America and have been translated into a variety of languages.

Warrens teaching was no less effective than his research. As a teacher of undergraduates, he packed the largest auditoriums, and his classes were known as tough but direct, incisive, and deceptively entertaining. He exercised his lyrical talents by setting historical events to well-known tunes. His devotion to teaching won him the Vice Presidency for Teaching of the American Historical Association, a National Teaching Award from the Danforth Foundation, and the UCSB Outstanding Faculty Teaching Award.