The battle of Mantinea, 362 BC.  

IXenophon gives no forces for the Thebans and their allies but Diodoros and Plutarch put them in the range of 50,000-70,000, realistically around 30,000 with the Spartans and their allies being a little smaller in numbers.

 The Mantineans,since the engagement was in their territory, occupied the position of honour at the right end of the allied line near the Mytikas ridge. Next came the Arcadians, then the Lacedaemonians , then the Eleans and the Achaeans in the centre, with the Athenians taking the left-flank position. The Mantinean and Athenian contingents fielded cavalry, guarding the flanks of the phalanx at the foot of the hills, and, unusually for a battle in this era, some Eleans cavalry was held in reserve.

Their number of hoplites may have been 5,000 Mantinean, 3 lochoi of Spartans about 1,500, 3,000 from Elis, 2,500 Achaeans, 1,000 Arcadians and 6,000 Athenians. There were about 3,000 peltasts mostly mercenaries,the majority Spartan paid and 1,600 cavalry which may have included all the Lacedaemonian cavalry which Agesilaus sent ahead with the 3 lochoi about 360, 500 Athenians, 400 Mantinean and 300 Eleans made up the numbers.

The Thebans held the left of their line, arrayed in a 50-deep block, then came the Tegeans and then the Argives. Next were the Euboeans, Locrians, Sicyonians, Messenians, Malians and Aenianians, and then the Thessalians and remaining allies. The  Theban cavalry was deployed on both wings, and their left wing also had Thessalian cavalry. The whereabouts of the Arcadian cavalry is unknown. The Theban cavalry was supported by hamippoi, footmen, which gave them the edge over their allied opposites.

The numbers may have been as follows : of hoplites 8,000 Boeotians, 5,000 Argives, 5,000 Arcadians, 3,000 Euboeans, 1,000 Locrians, 1,000 Sicyonians, 3,000 Messenians in all 26,000 hoplites. The Malians, Thessalians and Aenianians and mercenaries provided 4,000 peltasts and light infantry. There were 3,000 cavalry mostly Thessalian but probably 800 Boeotian with a few hundred Arcadians.

The Theban coalition advanced north up the road from Tegea, which is intersected by a few small streams; deployed to the west across the plain covered by a screen of cavalry which threw up much dust to obscure his intent and a force of hoplites to check any Athenian intervention, and gave the impression that he was ready to encamp and ground arms.

Seeing this, the Spartans and their allies went to their midday meal, upon which the Thebans rapidly attacked across the plain, their deep phalanx breaking the allies.

  During the course of the battle, the Mantinean cavalry was pushed back by the Theban and Thessalian cavalry, while on the eastern flank, the Athenian cavalry was defeated by the Theban cavalry, hamippoi and slingers deployed along the Kapnistra ridge, aided by Thessalian javelin men.

The Theban phalanx crashed into the allied phalanx, and helped by their victorious cavalry, broke it. At the moment of victory, Epaminondas was mortally wounded by a Spartan called Antikrates. Deprived of their leader, the coalition forces failed to pursue the defeated allies, and the greatest Greek battle to date came to in inconclusive end. On the Eastern flank, the victorious auxiliaries and peltasts had dispersed to plunder, and most of them were killed by the Athenians. At some point one of Xenophon's sons was killed. Both sides set up trophies as victors and both sides sent heralds as the vanquished to ask for the bodies of the fallen.

Here is Xenophons description

When the enemy saw them so unexpectedly approaching, not one of them
was able to maintain tranquillity: some began running to their
divisions, some fell into line, some might be seen biting and
bridling their horses, some donning their cuirasses, and one and all
were like men about to receive rather than to inflict a blow. He, the
while, with steady impetus pushed forward his armament, like a ship-
of-war prow forward. Wherever he brought his solid wedge to bear, he
meant to cleave through the opposing mass, and crumble his adversary's
host to pieces. With this design he prepared to throw the brunt of the
fighting on the strongest half of his army, while he kept the weaker
portion of it in the background, knowing certainly that if worsted it
would only cause discouragement to his own division and add force to
the foe. The cavalry on the side of his opponents were disposed like
an ordinary phalanx of heavy infantry, regular in depth and
unsupported by foot-soldiers interspersed among the horses.[14]
Epaminondas again differed in strengthening the attacking point of his
cavalry, besides which he interspersed footmen between their lines in
the belief that, when he had once cut through the cavalry, he would
have wrested victory from the antagonist along his whole line; so hard
is it to find troops who will care to keep their own ground when once
they see any of their own side flying. Lastly, to prevent any attempt
on the part of the Athenians, who were on the enemy's left wing, to
bring up their relief's in support of the portion next them, he posted
bodies of cavalry and heavy infantry on certain hillocks in front of
them, intending to create in their minds an apprehension that, in case
they offered such assistance, they would be attacked on their own rear
by these detachments. Such was the plan of encounter which he formed
and executed; nor was he cheated in his hopes. He had so much the
mastery at his point of attack that he caused the whole of the enemy's
troops to take flight.

But after he himself had fallen, the rest of the Thebans were not able
any longer to turn their victory rightly to account. Though the main
battle line of their opponents had given way, not a single man
afterwards did the victorious hoplites slay, not an inch forward did
they advance from the ground on which the collision took place. Though
the cavalry had fled before them, there was no pursuit; not a man,
horseman or hoplite, did the conquering cavalry cut down; but, like
men who have suffered a defeat, as if panic-stricken[15] they slipped
back through the ranks of the fleeing foemen. Only the footmen
fighting amongst the cavalry and the light infantry, who had together
shared in the victory of the cavalry, found their way round to the
left wing as masters of the field, but it cost them dear; here they
encountered the Athenians, and most of them were cut down.