This question hit while reading about the battle of Gaugamela the other day. I read that Alexander apparently had a reserve phalanx in the rear, protecting amongst other things, his camp.
Now to expand the question:
Nor english, so sorry if I'm using the wrong terminology. "Rearguard" might mean eomething else than i imagine.
Also, the time period I'm wondering about is antiquity, from Alexander to the rise of Rome, but I appreciate answers from other periods as well!
My period of expertise is Classical Greece, which is a little earlier than the period you're asking about, but it is the world from which Alexander's tactics emerge, so I hope this will still be helpful.
The typical battle plan involved the commitment of all available fighting men. Concerns about encirclement and breakthrough led the Greeks to make their battle line as wide and as deep as they could; additional troops were stationed on the flanks to protect the phalanx. Most battles were decided by a single clash of opposing formations of heavy infantry and cavalry.
The battle of Gaugamela is an exception. Alexander knew he was so badly outnumbered that he would never be able to extend his line far enough to avoid encirclement. He therefore decided to take part of his army out of the line entirely and to deploy it in reserve, ready to protect the back of the main phalanx when it was inevitably attacked in the flank by the enemy. The decision made sense in the circumstances; if Alexander's army had been notably larger, or the Persian army much smaller, Alexander likely wouldn't have bothered to keep any troops back from the initial clash.
Now, Alexander's tactics may have been unusual, but they were certainly not new. Nearly a century earlier, in 415 BC, when the Athenians fought the Syracusans, they were concerned that the vastly superior enemy cavalry would try to outflank and rout their army. Their solution was similar to Alexander's:
Half their army was drawn up eight deep in advance, half close to their tents in a hollow square, formed also eight deep, which had orders to look out and be ready to go to the support of the troops hardest pressed. The camp followers were placed inside this reserve.
-- Thucydides 6.67.1
There are many other Classical Greek examples of units of cavalry being used in this way, held back and ordered to protect the line and wait for an opportunity to attack the enemy at a point of weakness. During the March of the Ten Thousand, units of hoplites were also used as a reserve, with explicit orders to support any part of the line that was in trouble. However, these reserves usually numbered just a few hundred. The large-scale infantry reserves of Syracuse and Gaugamela - essentially a second battle line - were exceptions reflecting the circumstances of those particular engagements.