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The Whole North Into Gaul
Taking numbers and applying them to the Notitia Dignitatum is an even bigger challenge. First one must account for variations in unit size, and secondly account for copies, transfers, errors, destroyed units, and of course the date chosen. For example, the Comes Illyricum wasn’t created until possibly 402, and the Comes Britannium was created after 395, but prior to 405, and did not arrive in Gaul until 408. Knowing when a command was created also helps to judge where new field army units were drawn from. The arrangement of different units also presents different unit sizes, and then there is the question of units whose sizes can only be guessed at. Either way, using the numbers presented above, an analysis of the Notitia Dignitatum would show the Western Empire fielded about 233,250 men in both the Field Armies and Border Garrisons.
Such a number seems exceptionally large for the time of Aetius, and indeed it is. By the year 450, the remaining units in Britain, Africa, and Spain had all been destroyed, which would leave an army of about 142,450 men, of which 91,450 men would have belonged to the field armies. The loss of Africa, however, is known to have cut an additional 40,000 Infantry and 20,000 Cavalry; this would leave Aetius with 31,450 Comitatenses, and 51,000 Limitanei. The Romans tried to supplement the losses some, mostly by opening up trade with Greece, enforcing taxes on the Italian aristocracy, and passing a law specifically for the payment of soldiers called the Siliquatia; but they didn’t compare. Coello shows that most Roman units in the 3rd century operated from day to day at about 70% of their full strength, meaning that this force more likely numbered 22,015 Comitatenses and 35,700 Limitanei. Such a force would still be a formidable opponent to the Federates of Gaul, but did not compare with the 3 field armies Aetius had before.
Aetius seems to have enjoyed a practice of using a “half-Roman” army: it was Roman in discipline, equipment, training, and organization, but the soldiers were not recruited from ethnic Romans. His forces are described as “great” or “strong” from about 30 years of campaigning experience, and outright loyalty to a powerful and dedicated general. He led what seems to have been around 22,000 men to Chalons directly, with possibly another few thousand from local or retreating limitanei, bring the total Roman army at the battle maybe in the order of 24,865 men. These men had fought alongside the Huns, and certainly knew how to defeat their Germanic and Sarmatian adversaries. They would have been encouraged by the addition of federates to their cause, but that was a matter that took a little bit more than a simple “call to arms” from the venerable Genralissimo of the west.