the locality in which they were registered by the census (legal term is 'origo'). This measure had the effect of legally tying tenant farmers (coloni) and their descendants to their landlords' estates. Military infrastructure In parallel with restoring the size of the army, Diocletian's efforts and resources were focused on a massive upgrading of the defensive infrastructure along all the empire's borders, including new forts and strategic military roads. Constantine After defeating Maxentius in 312, Constantine disbanded the Praetorian Guard, ending the latter's 300-year existence. Although the instant reason was the Guard's support for his rival Maxentius, a force based in Rome had also become obsolete since emperors now rarely resided there. The imperial escort role of the Guard's cavalry, the equites singulares Augusti, was now fulfilled by the scholae. These elite cavalry regiments existed by the time of Constantine and may have been founded by Diocletian. Constantine expanded his comitatus into a major and permanent force. This was achieved by the addition of units withdrawn from the frontier provinces and by creating new units: more cavalry vexillationes and new-style infantry units called auxilia. The expanded comitatus was now placed under the command of two new officers, a magister peditum to command the infantry and magister equitum for cavalry. Comitatus troops were now formally denoted comitatenses to distinguish them from the frontier forces (limitanei). The size of the Constantinian comitatus is uncertain. But Constantine mobilised 98,000 troops for his war against Maxentius, according to Zosimus. It is likely that most of these were retained for his comitatus. This represented about a quarter of the total regular forces, if one accepts that the Constantinian army numbered around 400,000. The rationale for such a large comitatus has been debated among scholars. A traditional view sees the comitatus as a strategic reserve which could be deployed against major barbarian invasions that succeeded in penetrating deep into the empire or as the core of large expeditionary forces sent across the borders. But more recent scholarship has viewed its primary function as insurance against potential usurpers. (See Strategy of the Late Roman army below). Constantine I completed the separation of military commands from the administrative structure. The vicarii and praefecti praetorio lost their field commands and became purely administrative officials. However, they retained a central role in military affairs, as they remained responsible for military recruitment, pay and, above all, supply. It is unclear whether