The Late Roman Army

[Robert Vermaat]

Each of these areas, behind the Limes, has a small field army. This is true defence in depth, and at the same time very dangerous for these armies were very difficult to control, contrary to your argument.

Not exactly because those small field armies were no match for the comitatenses armies, contrary to the Early Empire, when the legions were spread over the limes, so they didn´t represent a real threat to the imperial government

Like razor rmarked, that's what the field armies were - the comitatenses. In fact, we see usurpers being lower and lower in rank. Magnus Maximus in Britain surely was a brigade general, a comes in charge of the British field army. Constantine III may have been one as well. These smaller field armies were the development from the original field army, a cavalry reserve in Italy under Galerius. But gradually the tactical need was for even faster reaction forces, meaning that field armies had to be even closer to the front. Of course, in time a field army was in great danger of becoming 'rooted' to the area where it had been fighting, and this seems to have happened in some cases, especially when the Limitanei had been destroyed.