The Makedonian phalanx -- why such depth?

[Paralus]

I may have the wrong author, but I believe Polybios describes the formation at Magnesia as appearing like two armies, one in front of the other. I'm inclined to think that is exactly what it was. So, one 16 rank phalanx as reserve behind another.

Yes, that would be Appian (Syriaca 7.35-37) edited below:

The total force of Antiochus was 70,000 and the strongest of these was the Macedonian phalanx of 16,000 men, still arrayed after the fashion of Alexander and Philip. These were placed in the centre, divided into ten sections of 1600 men each, with fifty men in the front line of each section and thirty-two deep. On the flanks of each section were twenty-two elephants. The appearance of the phalanx was like that of a wall, of which the elephants were the towers [...] The serried phalanx, in which he should have placed most confidence, on account of its high state of discipline, was crowded together unskilfully in a narrow space [... there follows the description of the wings, cavalry, chariots et al] The appearance of his formation was like that of two armies, one to begin the fight, the other held in reserve...

My reading is that he is describing the army’s appearance – “his formation” – as a whole, not just the phalanx which formed the centre. He is clear (like Livy both working from the same source – likely Polybios) that it was arranged in “ten sections” of 50 x 32 and thus Appian’s “crowded together” at the centre of the Seleucid array.

Appian makes plain the incredulity of Seleucus’ “friends” at Seleucus wasting the phalanx’s skills by crowding it together so

The Macedonian phalanx, which had been stationed between the two bodies of horse in a narrow space in the form of a square […]They accused him of his latest blunder in rendering the strongest part of his army useless by its cramped position..

This would be the third time Appian (or Polybios) has pointed out Antiochus’ error in cramping up the phalanx 32 deep. Evidently this is how it fought in the battle. Evidently the “friends” thought that divisions of 16 deep – stretching the line – will have far more useful.

Just on Polybios. Agesilaos points out to me (not from this forum), quite rightly, that his audience was in great part Roman. Romans, then, are expected to be thoroughly familiar with phalanx tactics and that 16 always “closed up” to eight so as to fight?

Perhaps that’s why Polybios felt the need to finish off his discourse on the phalanx – for those who already well knew(?) – with the already quoted line about the force of its charge when 16 deep?