How did infantry tactics change?

OK. I know this isn't completely to do with EB's timeframe, but what the hell.

Everyone who knows about Rome's history knows that it's military tactics changed constantly, and that there was a significant difference between how a Roman army in the 1st and 2nd century AD fought and how an army from the 4th and 5th Century AD fought. The Roman army by the late period was relying heavily on foreign mercenaries and Foederati troops and was more cavalry-orientated. Now, as the title obviously states, my questions deals with the infantry tactics of the time.

A typical Roman Legionary of the 1st and 2nd Century AD would be heavily armoured: He´d be wearing lorica hamata, squamata, or segmentata (along with a helmet), and carrying the large rectangular curved Scutum, two pila and a short gladius, designed for stabbing. Clearly this is a type of infantry designed to advance at a steady pace, absorb an enemy's charge, or just generally weaken enemy line troops before cavalry, or fresh troops, delivered the killing blow.

Now, the Roman legionary from the period of the fall of the western Roman empire (not Limitanei, who were more like militia) typically wore chainmail or scale armour, as well as a helmet (which was designed differently), and carried a large, yet flat oval shield and a longer sword designed more for swinging blows, he'd also (depending on the troop type) be carrying either a set Plumbatari darts or a light throwing spear.

Now, the equipment of the late Roman infantryman obviously shows a considerable change in infantry tactics of the time. I personally think that the late Roman legionary was designed as a more offensive troop type, whose job was to charge the enemy infantry to support the cavalry when it had engaged the enemy. The legionary from the Caesar's time period, by contrast, appears to have been a more defensive type of infantry designed to hold the enemy in place for flanking maneouvres.

That's what I think, though obviously I could be wrong. Can anyone here enlighten me on how Roman infantry tactics changed from one period to another? Were there any specific tactics which were preferred and, if so, why were they chosen over the ones that had been used previously? Obviously one can't always use the same strategies over and over so if the late Roman army relied on a (slightly) lighter, faster, and more offensive troop type, it must have been because it had proven to be effective, but why had the pre-christian Legionary ceased to be effective?

Basically this is a general 'why did change' question. Anyone here got any ideas why?

This is just a guess but the early post Marian legionaries were fighting mainly infantry based armies. The Successors' cavalry had declined in quality and quantity and they relied on the phalanx for offence, the Celts/Germans mainly used infantry and the legionaries were designed to fight these opponents.

The later legionaries were up against enemies who relied more on cavalry so back came the spear and a longer, sword is going to be more effective against cavalry than a short stabbing sword. The flat shield may have had something to do with economy as I would guess this was easier to mass produce than the curved rectangular shield.

G O'C

Yeah. Earlier, the infantry legions were the mainstay of Roman military might, but they were facing mostly infantry in the west. I think their encounter with eastern calvary tactics greatly changed this, and they began to employ in both cataphracts and horse archers to compliment the traditional infantry. In the late empire, if I'm not mistaken, the equipment changes I think were mostly due to pragmatic reasons. Segmenta was considerably more expensive to manufacture and maintain than chain and scale, same with the helmets. I think the spatha was adopted over the gladius hispanis and the lighter ovular shield over the scutum for 3 reasons if I'm not mistaken. 1. there were more and more "barbarians" serving in the legions and as foedrati, it seems to me it would make since to produce more standardized equipment, and these men would be used to traditionally longer swords. 2. Less time for training in the traditional Roman style with the more disciplined formations. 3. Infantry, while still playing a major role, was taking more of a back seat to calvary, thus contributing to #2.

I'm just trying to regurgitate what I've read, I can't remember where I read these things though, so I'm certain that some more intelligent member will come and refute what I've just stated. I think I read some of this in this thread over at the IBFD mod forum:

http://www.twcenter.net/forums/showthread.php?t=106375

one thing is not mentioned here - when the spatha took over the gladius the roman army was still overwhelmingly infantry based, yes cavalry was become more and more important (not difficult considering its status in the early empire) but still a support for the heavy comitatenses (post-diocletian and -costantine field troops)