How often did Roman v/s Roman battles maintain a disciplined formation and not end in a chaotic, free-for-all melee?
Also, how would soldiers distinguish friend from foe if everyone wore standard Roman armor?
Okay, that's a few questions. I'll try and take them one by one.
I am not sure what you mean by fog of war being thick in this context. You mean like literal fog limiting visibility? That might occur in the age of gunpowder warfare, when massive quantities of cannon and muskets discharging could indeed greatly limit visibility on a battlefield. But the Romans had no such weapons. At worst you'd get dust clouds from galopping horses and marching feet. These might be useful to spot an army at a distance, but would not be enough to really limit visibility on the battlefield.
In the more common figurative sense the Romans of course had very limited means of information gathering at their disposal. A general could not hope to exert a great amount of control once the fighting started, though, and mostly had to rely on his subordinates and junior officers to take good decisions on the spot.
This confusion could lead to all manner of tragedy. A famous story related by Plutarch illustrates this:
In 43 B.C. near the Greek town of Philippi, the army of Ceasar's assassins Brutus and Cassius fought a battle against the army Caesars heirs, Marcus Antonius and Octavian. (the future Augustus.) Brutus and Cassius divided the command between them. On the first day of battle, Brutus' army did pretty well. Cassius was beaten back, but all in all it was a draw. That's when tragedy occurred:
Thus, then, under compulsion, he [Cassius] withdrew with a few followers to a hill overlooking the plain. But he himself could see nothing, or next to nothing, of the sacking of his camp, for his vision was weak; the horsemen about him, however, saw a great troop riding up which Brutus had sent. But Cassius conjectured that they were enemies, and in pursuit of him. Nevertheless, he sent out one of those who were with him, Titinius, to reconnoitre. The horsemen spied this man as he came towards them, and when they saw that he was a trusted friend of Cassius, his intimates, shouting for joy, leaped from their horses and embraced him warmly, while the rest rode round him with shouts and clashing of arms, thus, in their boundless joy, working the greatest mischief.
For Cassius thought that Titinius was actually taken by the enemy, and with the words "My love of life has brought me to the pass of seeing a friend seized by the enemy," he withdrew into an empty tent, forcing along with him one of his freedmen, Pindarus, whom, after the disaster which befell Crassus,he used to keep in readiness for this emergency. From the Parthians, indeed, he had made his escape; but now, drawing his robes up over his face and laying bare his neck, he offered it to the sword. — Plutarch, Life of Brutus
So Cassius ordered his freedman to kill him all over a case of mistaken identity, and the next day Brutus lost the battle. Probably in part because Cassius' soldiers were none too happy that their general was suddenly dead.
You mean like you see in films, or television shows like Rome? Those did not happen. Soldiers would stay in their battlelines. These might become less disciplined during battle - we have sources telling of formations of men clumping so close together they could hardly fight anymore. But a chaotic, free-for-all melee would be murderous for all sides involved. Humans do not do such a thing. (Unless the fighting is fake and a movie director tells them to.)
There are several situations in which chaotic fighting might occur. Firstly, when one side is defeated or panics and flees. Then the lines would break up into many smaller groups. Individuals might run and hide. Small groups of soldiers might stick together to try and defend themselves. The victorious side would viciously pursue the losers and try to kill as many as possible. This would not by anything like a free-for-all, though. Mostly it would just be fleeing men being slaughtered or trampled underfoot. Ancient warfare was not a pretty thing.
The second might occur during the sack of a city or fortress, after the victors broke through the walls and started going through the town looting, raping and killing. Again, this is more like vicious murder of mostly defenseless victims than a free-for-all.
On battles the nature of ancient and medieval battles in general there are some very good answers in older threads such as this one: https://www.reddit.com/r/AskHistorians/comments/48pumt/did_the_people_in_the_front_lines_of_ancient/ with answers from u/Iphikrates and myself.
Since there would be no chaotic melee, most of the time it would be easy to tell friend from foe: your friends are the ones standing next to you. The enemy would be the ones in front of you. Besides that, Romans had their standards and the symbols on their shields to help identify which legion or cohort a soldier belonged to. Tacticus describes the following scene from the civil war in the Year of Three Emperors:
A ballista of enormous size belonging to the Fifteenth legion began to do great harm to the Flavians' line with the huge stones that it hurled; and it would have caused wide destruction if it had not been for the splendid bravery of two soldiers, who, taking some shields from the dead and so disguising themselves, cut the ropes and springs of the machine — Tacitus, Histories
Emphasis mine. Although as this story shows... symbols and shields could also be used to trick the enemy. False flag operations are as old as dirt, apparently.
Still, battlefields are chaotic and confusing, and accidents could happen. Even if they knew who their immediate comrades were, soldiers might mistake another friendly group of soldiers for the enemy. Caesar at one point attempted to surround a Gallic army by sending a detachment of his troops to seize a hill. Only then his scouts reported that the hill was occupied by the enemy and he had to retreat. And then he found out that his scouts had been mistaken: they had mistaken the Roman detachment for the Gallic army. And that was not even a civil war, but a war against a different people.
At the battle of Delium, it was even worse: there the Athenian army surrounded a flank of the Boethian army. But if you surround the enemy, there are no longer two orderly lines facing eachother... and the Athenians ended up attacking and killing eachother.
Thank you for your answer but I definitely didn’t mean actual fog (and certainly not fog produced by gunpowder in a battle that occurred roughly 1000 years before it was invented). The metaphorical term “fog of war” was coined by von Clausewitz. From wiki:
The fog of war (German: Nebel des Krieges) is the uncertainty in situational awareness experienced by participants in military operations.[1] The term seeks to capture the uncertainty regarding one's own capability, adversary capability, and adversary intent during an engagement, operation, or campaign. Military forces try to reduce the fog of war through military intelligence and friendly force tracking systems.
It’s basically a term that refers to the confusion that ensues during a battle.
As for a chaotic free-for-all melee, whereas I understand that the strength of the Roman legion relied on its discipline to maintain formation, many Gallic tribes (Helvetii, Cimbri, Seuvi, etc) lacked this vital military doctrine. Hordes of militarily undisciplined “barbarians” would charge head on en mass into the teeth of the well disciplined Roman war machine, often leading to self-decimating results.
Just to confirm that I’m understanding your explanation correctly, you’re saying that all Roman v/s Roman battles consisted of both sides maintaining formation and battle discipline?