So, having correctly guessed the Higgs announcement, I'll revert to normal. Let's talk about the army.
If you were a guy in classical Athens, and your father was a citizen, then when you turned 18 you joined the army. No exceptions. You were in for 2 years, which was boot camp. Boots were known as ephebes. Nico protests on several occasions in the books that he's done his time as an ephebe. That's his way of saying he's paid his dues and though he might look young, he's a for-real citizen of Athens with all the rights and privileges that implies.
Athens had its equivalent of the stereotypical boot camp sergeant. The ephebes were grouped by their tribe membership, and each group had in charge of them a man over forty years of age; someone who was voted by the citizenry to be a fine, upstanding examplar to the lads, and likely to instill the necessary virtues. Which meant he was really, really tough.
Ephebe training was as much about instilling moral character as toughening you up and teaching you how to fight in a phalanx. After the first year, you were given a spear and shield by the state and swore an oath.
It seems the ephebes spent a lot of time on guard duty. They also patrolled the countryside. I suspect phalanx training was drilled mercilessly, because the last thing you need in a battle is some idiot pointing his spear the wrong way.
Phalanxes work like this: there are three rows (probably). The younger men at the front, then the middle-aged, then the old men at back. The phalanx is a very dense rectangle of men. There's no such thing as individual combat. When properly lined up, a man has his spear in his right hand (tough luck for the lefties). He has his huge, round hoplon shield on his left arm. The shield covers the left half of his own body, plus the right half of his left-hand neighbour's body. His right-hand neighbour's shield in turn covers his right half. This overlap is a natural consequence of everyone holding a big shield in their left hand, but stop and think about it for a moment, so you'll see what happens next...
When a phalanx charges at the enemy, it runs on an ever-increasing angle to the right. Why? Because everyone depends on their right-hand neighbour for some of their shield protection. Everyone wants to nudge themselves a little more behind their neighbour's shield, and the neighbour is busy doing the same.
Generals know this will happen and have a fair idea how much drift there'll be. They position the units to account for it.
It's not such a bad idea anyway, because although there's a lovely row of shields down your left flank, down your right flank there's absolutely nothing but exposed right arms. This is why the right hand side of the line is considered the position of honour. You only put your best soldiers there. If your right flank gets enveloped, it's going to be a bad day.
Once contact is made, the whole thing turns into a pushing match. Think rugby scrum with sharp implements and you're probably fairly close. The idea is to break the enemy line, not kill the individuals. Once a line is broken, the soldiers are utterly exposed without their shield wall and will typically run.
Warfare is normally a citizen-only exercise, but interestingly, the Battle of Marathon is believed to be the first battle in history in which the slaves fought alongside their masters.