Civil Wars question...

[Crispvs]

It may be worth remembering that there were several periods of civil war in Rome's history.
During the late Republican period there was the war between Marius and Sulla, and later of course Caesar's civil war, followed soon after by the wars between the triumvirs. The next period of civil war (not including Vindex's revolt) was that which followed Nero's deposition. After that Rome was free of civil war for over a century but civil war was a frequent occurrence for around sixty years in the middle of the third century AD and then there were several further periods of civil wars in the fourth century AD following the death of Diocletian and the breakup of the tetrachy.

As these wars are spread over a roughly five hundred year period, there would have been major changes in how soldiers might recognise friends and enemies on the field of battle. Over time shield blazons and standards changed and developed and methods of recognition methods probably changed and developed with them.

As far as I know there is not much evidence of Roman shield blazons prior to the first century BC. What few indications there are might suggest that prior to the first century BC, with the Roman army being effectively a citizen army raised in times of need, where soldiers provided their own kit, any shield decoration would be down to personal choice.
It has been plausibly suggested that Roman unit blazons developed as a direct response to the need to recognise friendly units during periods of civil war. Prior to Marius different legions had different standards but by the time civil wars came about, all legions carried the eagle, which must have increased the need for another way to recognise units. The first proof we have for distinctive unit shields is Tacitus' reference to soldiers using shields taken from fallen opponents to get close to the enemy during the second battle of Cremona in AD68. However, there is sculptural evidence for different shield blazons which are probably unit blazons dating to the second quarter of the first century AD. Vegetius mentions that (at some undefined time previous to his own) soldiers had their centurione's names painted on their shields. If Vegetius is correct in this detail, it is possible that this was an early form of unit recognition which may have predated the development of distinctive unit blazons.

Another thing to consider is helmet crests. Caesar mentions that his fifth legion were known as the 'larks' due to the distinctive cresting arrangement they used on their helmets. There is also evidence to suggest that the two Adiutrix legions raised from marines a century later in the late AD60s could be distinguished by their use of copper-alloy helmets mounted with a central crest and side plumes. This sort of unit identifier might have been far more common than the current state of the evidence gives us proof for.

The other thing to consider is standards. Each century had its own standard and there is some evidence (specifically for both the Fourteenth legion and the Praetorian cohorts) for units using distinctive totemic images on their standards, which would have made them distinctive. There have been suggestions that the varying number of discs shown on images of signum standards indicate particular centuries with cohorts and it is possible that some of the other items or colours on signa may have indicates particular cohorts. Thus we have the possibility that an individual signum could have told an observer exactly which legion, which cohort and even which century was facing him on the field of (civil) war. Signa had a long history and were in use at least as early as the first century BC and were still in use during the third century AD and possibly later.

Very little is known about the development of shield blazons or helmet crests during the second and third centuries AD but by the early forth century much had changed and shield blazons appear to have undergone a wholesale change (or maybe many changes). Still, we have only a paucity of evidence for this period and it is not until the compilation of the Notitia Dignitatum in the late fourth century AD that we get an illustrated list of unit blazons, although thanks to medieval copying practices we have no way of knowing how accurate the depictions in the surviving copies of the Notitia Dignitatum are, although it does provide evidence for late fourth century AD units have distinctive blazons.

During the third century AD the Sarmatian derived 'drako' standard began to make its way into Roman usage. Only one example has so far been positively identified in the archaeological record (although the so-called Deskford Carnyx may actually have been a drako as well) but it is possible that units were able to bear unit distinct carnyces by the use of either particular head forms or distincively coloured tails and/or poles.

As David says, in field combat situations (as opposed to small unit or guerrilla actions) units would hold their own places in the line of battle and would almost certainly already be aware of which units were included in the enemy's force. In addition, during the pre-battle deployment and any standoff periods, soldiers would probably note the appearance of their opponents' standards, shield blazons and cresting, allowing them to determine the visual differences between enemies and friends.

There is also evidence for particular cultural practices such as the salute to the sun which Tacitus mentions one unit performing at Cremona. Units may also have had distinctive songs and chants.

I hope this is of some use to you.

Crispvs