Quote Originally Posted by CirdanDharix
... Of course, that's an old problem--during the Peloponnesian war, the Athenians had trouble distinguishing their Dorian allies from their Dorian enemies, because they both sung their battle hymns in Dorian.
Good get, I was thinking of the same episode: Dorian Allies of Athens outside Syracuse IIRC, singing the paean.

Generally soldiers knew on another as part of a polis, kinship group, tribe whatever.

They definitely rallied around standards and known leaders.

As mentioned flankig scenarios (and I suppose night battles) were quite terrifying affairs with every shape looming out of the mist a potential foe.

There's an hilarious scene in Asterix the Legionary where Caesar and Sciopio's men mill around in battle. By the time of the civil wars it would'vebeen a real problem in melees, but the romans were pros and no doubt they came up with badging or colour solutions.

Friendly fire was a massive problem down to today. At Shiloh the New Orleans arty unit wore resplendent French-style blue uniforms but they had to turn them inside out to show the white lining so their grey and dun reb pals would stop gunning for them. Lemon Lips Jackson bought a friendly ball in the leg at Chancellorsville.

The C18 UK had green uniforms (light inf), bluejobs, rainbow coloured cavalry. The French were the whole dice and box, so were the Russians. There were some regular rank and file colours (eg prussian Blue for Prussian line inf) but the cav in particular were flamboyant/variable in the extreme. There was a Napoleonic French cav tactic to ride up along the enemy line through the smoke of battle in column as though they were friendly; ordinary inf would think they were just another fancy-dress cav unit, and they weren't attacking so they must be friendly. They'd slip in behind the line and wallop. Don't know if it ever worked though.