A couple of points.
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willowfall
At the time of the Battle of Teutoburg Forest (9CE), the Roman Legions were transitioning from mail (lorica hamata) to the Lorica Segmenta.
A Roman soldier - despite what you see in movies - avoided one on one sword fights.
Thanks Willowfall for the points you raised, I did mean to get back to you sooner. Naraku also once again thank you for your in put.
As to the Lorica Segmatae, it's use even within the legions never seems to have become universal. While Rome seems to have developed several centres of production across the Empire it was never even close to the kind of industrialisation that modern commentators take for granted. Segmented armour seems to have been an effort to gain the protective qualities of plate without using expensive bronze which contrary to popular opinion, including mine for many years, was actually stronger than existing grades of iron for centuries to come.
It was always expensive, being both time consuming to produce and it had to be produced by craftsmen and difficult to keep in good repair. Hence after the Third Century AD as central logistic support declined it went out of fashion.
Chain-mail is likewise not exactly easy to produce but it is easier to do than any form of plate. Modern re-enactors frequently use wire coat hangers as their source of iron wire
Properly cut and pinned or abutted it does give a fairly decent level of protection though smiths who produced the real deal for military purposes likely used better grade metal.
Now as to the point of formation raised by Willowfall and Naraku. The Romans benefited from adopting a fairly unique formation. According to available source the Romans seem to have realised that in order to move a large body of men in close order you actually needed to leave roughly man size gaps between each man.
The phalanx and the shield wall formations that you see in most cultures of the same and later periods were ideal for low skilled and erratically trained militia. Yet the Romans (who started with militia armies recall) adopted a formation that could only have required the utmost discipline and drilling to maintain.
The famous scutum shield may have contributed to this. In the examples found the arrangement of the hand grips is unusual compared to most forms of shield as it would position the arm down the length of the shield. Now this might be a reaction to the fact that Roman style scuta weigh in the region of 10 kilos or 22 pounds but it would have been awkward to use facing the enemy in an open fight. On the other hand in close formation the level of protection provided to yourself and to a lesser extent those around you, is massive.
By and large throughout history most armies do not appear to have interpenetrated as seen on TV and film but used some kind formation even if only the mob of goons with the bravest at the front in actual contact with the enemy and more folks pushing up behind so they could not run away and actually had to fight.
The Romans just worked out the best way to get their mobs into contact with the foe. At that point the gladius comes into its own being perfect for lethal stabbing strokes in a confined space.
The auxiliary infantry mostly seem to have used the same general tactics as the legions but with slightly different weapons though we have to be careful in assuming degrees of difference because the archaeological record is not conclusive and the written sources even less so.