Late Roman Army - Evolution of The 4th-century Army - Diocletian - Manpower

Manpower

To ensure the army received sufficient recruits, Diocletian appears to have instituted systematic annual conscription of Roman citizens for the first time since the days of the Roman Republic. In addition, he was probably responsible for the decree, first recorded in 313, compelling the sons of serving soldiers and veterans to enlist.

Under Diocletian, the number of legions, and probably of other units, more than doubled. But it is unlikely that overall army size increased nearly as much, since unit strengths appear to have been reduced, in some cases drastically e.g. new legions raised by Diocletian appear to have numbered just 1,000 men, compared to the Principate establishment of ca. 5,500 i.e. the new legions may have increased overall legionary numbers by only ca. 15%. Even so, scholars generally agree that Diocletian increased army numbers substantially, by at least 33%. However, the only extant ancient figure for the size of Diocletian's army is 390,000, which is much the same as that of ca. 130 under Hadrian and well below the peak figure of 440,000 under Septimius Severus. The apparent contradiction may be resolved if one accepts that Diocletian was starting from a much lower base than the Severus figure, as the army's effective size had probably shrunk sharply as a result of losses from plague and the military disasters of the late 3rd century. In this case, simply restoring numbers to their 2nd-century level would have involved a major increase. (See Army size below).

Some scholars estimate that the Tetrarchan army was significantly larger than the Severan army. Warren Treadgold argues that Ioannes Lydos' figure of 389,704 soldiers and 45,562 sailors accounts for the army in 285, while Zosimus' figures totalling 581,000 soldiers account for the army in 312. Treadgold estimates that the army size was roughly constant between 235 and 285, with rapid increase between 285 and 305, and was again roughly constant between 305 and 395.