The Makedonian phalanx -- why such depth?

[Justin of the New Yorkii]

Okay, question then from you guys. It is often said that Philip II "learned" a lot as a Theban hostage and applied this knowledge to his own phalanx many years later. Then what exactly did he learn and apply if the Macedonian phalanx fought in a depth of 8 men, a similar depth to the old-school phalanx employed by the city-states? I mean, wasn't the chief innovation of the Thebans a deeper phalanx used to smash the typical 8 or 12-deep formation or am I missing something?

I think the chief innovation made by Epaminondas was principally in the concept of force concentration; remember that at Leuctra, he had only about 7,000 levy hoplites facing an enemy force of over 10,000 hoplites, so he needed to devise a way to win the battle without asking too much of his lesser troops.

Most of the old phalanxes would have the weakest hoplites on the left side, and the strongest on the right, so that the lesser troops would be afforded greater protection from their shields. Epaminondas knew this was how the Spartans would be deployed.
So instead of using his weakest forces on the left (opposite Cleombrotus and his crack troops), Epaminondas deployed the Theban Sacred Band on the left in a super-deep phalanx, with his lesser troops staggered in echelon to the right.

He rolled the dice, hoping to destroy Cleombrotus' bodyguard as quickly as possible so the rest of the Spartans would rout before the lesser Thebans on the right got overwhelmed.



The Sacred Band actually took pretty severe losses at Leuctra during the initial clash, but Cleombrotus was killed and the Thebans were clearly winning, so they wheeled right and rolled up the Spartan line like a carpet. The lesser Thebans on the right didn't have to sustain contact for long at all.
Alexander's deployment at Gaugamela was similar in principle; he was outnumbered, so he went aggressively right at Darius' position, hoping to a knockout blow quickly and end the battle before the Persian numbers could tell.