The Late Roman Army

[Robert Vermaat]

The thing about cavalry is that it is tactically fast, not necessarily strategically fast. Horses are finicky creatures and don't like being pushed to their limits. Unless you have strings of remounts for every man, you can't spend the entire time in the saddle, and unless you have prepared fodder dumps, you need to spend much time and effort finding stuff for the mounts to eat that could otherwise be spent marching. Once the first few days are over and the horses need regular rest periods, a well-organised infantry column with a working supply train can actually outpace a cavalry unit.

Whatever reasons the army might have had for making the comitatenses cavalry-heavy (and there are good ones, not least that it is much easier to get off a horse you have than to get on one you don't have), I don't see improved strategic mobility featuring majorly.

Calton, I agree completely!
It's a common misconception to think of cavalry-heavy forces as 'mobile field armies'. I've read (Junkelmann comes to mind) that even until the First World War, infantry outpaced cavalry when marches took longer than 7 days in a row.
Cavalry is fast, but that's battlefield speed and impossible to keep up without remounts.

On of the reasons for large cavalry forces to stay spread out in the hinterland is the impossibility to kep large cavaly forces all the time at one point - the grass and fodder would run out soon.
Of course the other reason is that it's much better positioned there (the hinterland) as a reaction force.
The 'mobile' is a logical result of both.