This period's not my specialty but may I suggest the Stele of the vulture of the Mesopotamian Early Dynastic period (c.2450 BCE) which shows the ensi of Lagash, Eannatum defeating his opponents. One fragment shows a phalanx of shielded soldiers trampling over the bodies of his enemies.

So it kind of depends on your scope and rigor. Cemetary 117 dates from ~14,000BCE, and no that's not a typo. It's a big ol' pit of bodies we found in the Sudan, with several of the 59 bodies being clear victims of violence (some rather extreme). Some question whether it was a battle, saying that we haven't been able to demonstrate that the bodies all went in at the same time. Others point out that half the bodies are female and question whether or not that would happen in a battle, which I personally think is narrow-minded projection of modern norms into the past, but nonetheless all we know is that there are some bodies, some of which got stabbed and shot a lot.

If we're willing to go quite small but want more detail, Ötzi was involved in a clash that involved atleast of 5 people (himself, two companions, and two people he probably killed himself) in the Alps in around 3300BCE. We know this one from blood analysis of the Ötzi's body, including his clothes and weapons.

If you're willing to be fast and loose with your sources but really want the written sources, the Kurukshetra War "took place" around 3102BCE, or 5561BCE, or what have you. It's the war that is the subject of the Mahabarata, adn whatever the literary, religious, and cultural merits of the poem, I'm unwilling to put any money down that the battle happened any time near that (the poem was written around 400BCE), but nobody else seems to be even claiming that age.

Agrippa already brought up Mesopotamian Warfare, but probably the oldest battle that is 1)detailed 2)documented 3)large would be Khadesh, which is pretty fascinating for a number of reasons. The battle was a draw, but the negotiations after the battle basically turned it into a Hittite victory, and at the same time the only thing that the Egyptian leader needed out of it, looking like a strong military leader, he got out of it.

Well, it might not be the most ancient battle we know of, but the Battle of Megiddo is one of the earliest battles that we can date with a great deal of accuracy. The Egyptians kept a relatively detailed record of the battle and included the date, which has been equated with a day in April, 1457 BC. The battle was fought outside Tel Megiddo (in Israel) between an Egyptian army commanded by Thutmose III and a coalition of Canaanite states.

The Canaanite army anticipated the Egyptian invasion and positioned itself by the most likely route the invaders would take. However, Thutmose took his army around the Canaanites and through an unguarded mountain pass (which was so narrow that his men could only pass through it single-file) that brought them out behind the Canaanite forces, between them and Megiddo. The Egyptians routed the Canaanite forces but wasted valuable time ransacking the Canaanite camp, which allowed the defeated army to retreat behind the walls of Megiddo and forcedthe Egyptians to besiege the city for several months.