Constantine the Great, after his accession, reformed the command structure of the Army. Though he retained Diocletian's divisions of limatanei and comitatenses (respectively, the fixed armies of the border and the mobile armies of the interior), he placed all troops under the command of a magister peditum (L. Master of Foot) and a magister equitum (L. Master of Horse), wishing to reduce the power that the Praetorian prefect wielded over the armies.
This arrangement was duplicated in the East and West, being retained until Theodosius decided to mix everything up yet again, doubtless after the lessons learned from Valens' defeat at Adrianople. Having seen the power of the Gothic, Alan (Alanish? Alanic?), and possibly Hunnish horsemen (see footnote 7 over here, or see Ammianus Marcellinus XXXI), Theodosius wanted to increase the numbers and relative importance of cavalry in his forces. (This was a major break with the long tradition of Roman soldiering, which had always emphasized the importance of disciplined infantry. And also, legbagede sez:re - the emphasis on footment, I was actually just reading in Livy how the consul marching against Hannibal at Cannae actually had the cavalry dismount to meet the Carthaginian mercenaries...seems they just used the horsies to move about in the early Imperium and only chumps went into battle riding : ))
Theodosius abolished the division between Masters of Horse and Masters of Foot in the Eastern Empire; instead, he set up five masters of combined armies, two magistri militum in praesenti commanding the Palatine soldiers (Constantinople's rough equivalent of the Praetorians), three commanding the field armies in Illyricum, Thrace, and the East.
Over in the western half of the Empire, things were different; the old structure of cavalry and infantry remained intact, but the magister peditum would gain (largely thanks to the successes of Flavius Stilicho) greater and greater importance, eventually becoming magister utriusque militiae, or Master of Both Services. Though they were in theory subordinate to the Augustus, in practice the holders of this office became rulers in truth, if not in name, of the Western Empire. The rebellious Frankish general Arbogast, who attempted to overthrow Theodosius in favor of the puppet Eugenius*, was among the first magistri utriusque militae**, and also among the first to realize that hey, the army obeyed the magister, and the army was the true source of power in the Empire, so why not take over?
*He failed, being defeated by Theodosius and Stilicho in the Battle of the Frigidus. Eugenius was killed in the battle, and Arbogast committed suicide shortly afterwards.
**He was at first magister equitum, but came to command both branches; his title of magister utriusque militae was not official.
- J. B. Bury, History of the Later Roman Empire from the Death of Theodosius I. to the Death of Justinian (A. D. 395 to A. D. 565); (Macmillan and Co., Limited, 1958)
- John Michael O'Flynn, Generalissimos of the Western Roman Empire (University of Alberta Press, 1983)
- The Notitia omnium Dignitatum et administrationum tam civilium quam militarum, at http://www.fordham.edu/halsall/source/notitiadignitatum.html