Philip might quite likely have died on the field that day even though his army was victorious. Had that happened, his heir was close at hand.
Alexander, though, could also have died that day (in spite of his many babysitters). And the best thing to do, under such circumstances, would have been to not let Alexander take part in the battle at all. Not only would it have guaranteed Alexander's survival, in case of Philip's death, but it would also have let the generals on the left wing focus on the battle, rather than have to spend half their energy watching over Alexander.
And regarding “close at hand”: He didn’t need to be on the battlefield. Had Philip died, and had the battle been lost, then the battlefield would have been the worst possible place for Alexander to be. Had Philip died, and had the battle been won, then the Macedonian army would have been just fine without Philip’s teenage son “close at hand” to lead them. Parmenion – or some other commander – would have led them.
What source underpins your claim that Philip placed Alexander in command of the left wing? Diodorus, our only real narrative guide, states that "on one wing the King posted his son" whilst adding that "He stationed alongside him his most important commanders..." Justin does not mention him at all until the diplomatic mission to Athens to secure the surrender and "alliance" of the city. I'm with Marcus here.
I don’t read Greek. I can only rely on the translations. This is what Diodorus writes:
The armies deployed at dawn, and the king stationed his son Alexander, young in age but noted for his valor and swiftness of action, on one wing, placing beside him his most seasoned generals, while he himself at the head of picked men exercised the command over the other; individual units were stationed where the occasion required.
Philip, apparently, “exercised command over
"the other" wing. It certainly seems to me that Diodorus is saying, that while Alexander was in command of one wing, Philip was in command of the other.
I’m skipping the part where Diodorus talks about the Greeks, and how the battle was “hotly contested.” Then:
Then Alexander, his heart set on showing his father his prowess and yielding to none in will to win, ably seconded by his men, first succeeded in rupturing the solid front of the enemy line and striking down many he bore heavily on the troops opposite him. As the same success was won by his companions, gaps in the front were constantly opened. Corpses piled up, until finally Alexander forced his way through the line and put his opponents to flight.”
“Ably seconded by his men”. This would indicate to me, again, that Alexander was in command.
Diodorus continues:
Then the king also in person advanced, well in front and not conceding credit for the victory even to Alexander; he first forced back the troops stationed before him and then by compelling them to flee became the man responsible for victory.”
“…not conceding credit for the victory even to Alexander”. What would Philip have seen on the right wing? Probably the left wing advancing, and the Greeks fleeing. But I doubt he saw Alexander himself. If someone else was in command...why would Diodorus write “not conceding credit for the victory even to Alexander” when Philip under such circumstances would have had no reason to suspect that Alexander had anything to do with putting the Greeks to flight?
I agree with you and Marcus that all this might be propaganda, as Alexander not taking part in this particular battle at all to me would make a whole lot more sense. But it’s at least clear to me that
Diodorus himself thought that Alexander commanded the left wing, and that it, for that reason, was to him that Philip did not want to concede credit.
I disagree that this would have been the right time and place to experiment by letting Alexander command the left wing (or command anything at all) – however many capable generals that were there with him. This was the battle that would decide everything. Philip should have put the most capable person in every position, and let all his generals focus on
winning the battle, rather than on babysitting his son. Alexander should not only not have commanded the left wing: he shouldn’t have commanded anything at all, or even been on the battlefield.
But then again, maybe he wasn’t?