You know how, at various points in The Return of the King, Merry and/or Pippin will shout “for the Shire!” or “for Frodo!” before running to battle and certain death? Did ancient warriors really do this or is it artistic license?
Cry God For England, Harry and St George!
If it’s just artistic license, it’s certainly old artistic license. It’s even in Homer.
Lots of historic info here: Battle cry - Wikipedia
Including ancient Greeks mimicking owls in their battle cry.
ETA2: In eras in which the primary determination of who won the battle was often who was feeling brave enough to run headlong to death and not turn tail and run away, it makes sense that a loud, rousing cry of inspiration would be a standard part of the battle plan.
Exactly this. It would have been far more common than not. Battle, especially hand-to-hand, is terrifying and rallying cries/battle cries both helped internal esprit de corps and hopefully intimidated the enemy. Like rival fans trying to drown each other out at a football championship.
You’re a peaceful farmer taken from your land and family and given a spear. Suddenly you find yourself lined up with neighbors facing a horde looking to kill you and your entire way of life. You walk forward, for the first time in your life you are about to stab multiple people to death up close if you DON’T die.
I would shout so many things, so loudly.
I’m pretty sure that’s every era of war, if not necessarily every mode. The guy in a hardened bunker piloting combat drones likely does not have a battle cry, but the poor infantry mooks he’s dumping thermite on probably do.
Your wiki links includes “Slava Ukraine” as a modern battle cry, for example. Also, “Leroy Jenkins,” which I’m willing to bet has been shouted in an actual war at least once at this point.
The Anglo Saxons at the battle of Hastings shouted “UT! UT!” (out! Out!).
The CSA famously used their rebel yell.
Even in the US Civil War, the Confederate soldiers had the "Rebel Yell’
Show me your War Face! You got a War Face? (from Full Metal Jacket, a Vietman movie)
She cried “more, more, more”.
Besides morale, there’s the issue that for most of history there wasn’t much in the way of military uniforms. Screaming “For King X!” helped identify you as somebody the other people fighting for King X weren’t supposed to kill.
Not just the human voice alone:
And even more recently, the Japanese had “Banzai!” in WW2.
The Spanish battle cry of “Santiago!” was used by the Conquistadors in the Americas.
“Do you want to live forever?” Yes, Sarge, I do.
The etymology of the Rebel Yell is all over the place, but all the recordings I’ve heard have all sounded like a high pitched ‘Yip’ or ‘Yelp’. Wherever it first came about and why, by the ACW it sounds exactly like how you’d call hunting dogs. Which makes sense to me, a familiar sound that comforts your allies (“Hey, we do this all the time back home”) and scares the hell out of your enemies hiding behind trees.
It’s used as a war cry, but has deeper elements I think.
Here is an example
I kind of like that song but if the woman is doing a Confederate war cry I think it kind of ruins it.
It made more sense than trying to enrage American soldiers by yelling “To hell with Babe Ruth!”.
The clans of Scotland had their own war cries. The fiercest is Clan Cameron’s,
Chlanna nan con thigibh a’ so 's gheibh sibh feòil! — Gaelic for “Sons of the Hounds, Come Hither And Get Flesh!”
Basically telling their enemies that they’ll killed and fed to the Cameron dogs.
Too long, hard to harmonize into a concentrated roar during a mass charge. Everybody yelling with a different cadence - just ends up sounding like gibberish crowd noise. Fail, Camerons.
Compare that to the Mackintosh “Loch Moigh!” - so much more economical. Is it any wonder the Mackintoshes were more regularly on the winning side of their 300-odd year feud? No - no, it is not 
.
“Santiago y cierra España!” IIRC
ETA, I googled it and I had in fact RC.
The funny thing is that it sounds at first like saying “Santiago and Spain closes”, which has caused confusion and hilarity for longer than one would think (Sancho Panza asks Don Quijote if Spain is open that must be closed).
After a bit of thinking the real meaning is clear “Spain closes” means that the Spanish troops are to charge (to “get close” or “close the distance” with the enemy).
Dan Carlin’s podcast, recently on Alexander’s rise to power, the army his dad Philip left for him, and his conquests in Anatolia mentions Alala, which was the personification of the war cry - probably because the Greek war cry sounded something like “Alalalala!!!”.
It’s interesting because I feel like if you put a bunch of hoplites or pikemen shouting “Alalalalala!” while they charge their enemy in a movie about Alexander nowadays, you might be accused of anachronistically copying native American war cries for your Greeks. That style of war cry is probably pretty ubiquitous across many cultures, it seems like a pretty natural way to yell if you’re not chanting a phrase.