At the time of the Persian Wars the great mass of a Greek army was composed of armored foot soldiers with a thrusting lance about two meters long, the hoplites. The protective equipment consisted of helmet, harness, greaves, and shield. A short sword was an auxiliary weapon.
The hoplites form a tight tactical unit, the phalanx. The phalanx is a continuous linear arrangement composed of several ranks. The depth varies; very often we hear of an 8-man depth, which seems to be regarded as a kind of normal formation; but we also hear of 12-man and even 25-man depths.
In such a phalanx two ranks at most can participate in the actual combat, with the second rank stepping into the holes of the first at the moment of contact. The following ranks serve as immediate replacements for the dead and wounded, but they exercise principally a physical and moral pressure. The deeper phalanx will defeat the more shallow one, even if on both sides exactly the same number of combatants actually manage to use their weapons.
But for the advantage of this pressure, it would be much better to lengthen the line, outflanking the enemy and enveloping his two flanks at the moment of shock. But with equal opposing forces such an envelopment can only take place at the expense of the depth of the formation, and although it requires only a few minutes from the first contact of the two lines until the envelopment has been completed, nevertheless in this time the deeper of the opposing phalanxes would presumably already have overrun the shallow center of the opponent and would thereby have broken up the whole formation.
Therefore, in any consideration of the phalanx two principles stand diametrically opposed: depth, which gives weight, and length, which facilitates envelopment. It is up to the commander to determine the depth and length of his phalanx from the circumstances of the situation, the strength of the armies, the quality of the troops on both sides, and the form of the terrain. A very large army is more strengthened in the dimension of depth than in the dimension of length, because it is extremely difficult to move a long line forward in a fairly aligned and well-ordered way, whereas the formation of a deep column is not so easily disrupted.
Since the rearmost ranks of the phalanx almost never arrive at the point of using their weapons, it might appear superfluous to supply complete protective armament to all the warriors from about the fourth rank back. Nevertheless, we have no account from the Greeks to the effect that such a distinction was ever made. An unarmored person is not capable of really fighting against an armored one. The forming up of several ranks of unarmored men behind the armored ranks would therefore have been not much more than a kind of pretense. The realization that they could not really expect to receive any true support from these rear ranks would have seriously weakened the drive, the forward thrust of the foremost ranks, in which, of course, the value of the rearmost ranks normally lies. If, at any section of the line, it really happened that, by some possible chance splitting of the phalanx, the armored enemy penetrated into the unarmored rearmost ranks, the latter would have had to give ground at once, and the flight in this one area would easily have pulled the entire army back with it.
Least of all, then, would it have been desirable to put possibly unreliable men, slaves, in the rearmost ranks of the phalanx. They would do no good there but would be able, through pre mature, perhaps even malicious, flight, to create a panic quite easily, even among the hoplites.
This explanation does not eliminate, of course, the opposite proposition, that when one has some men less well armed, they are placed in the rearmost ranks. Such lightly armed or only partially armed men can also be useful by helping friendly wounded soldiers and by killing or taking prisoner those enemy wounded over and around whom the battle is being waged. Those are only secondary services, however, and the phalanx as such presupposes the most completely armed warriors possible throughout all the ranks.
Of the utmost importance in this kind of combat is the type of men who stand in the first rank. Again and again, in his war songs, Tyrtaeus praises the men of the forward battle, 'among those fighting in front.' The later theoreticians recommend to army commanders that the most reliable men be placed in the first and last ranks, in order to hold the entire phalanx together. An accused Athenian citizen brought out in his defense in a trial the fact that he had voluntarily had himself placed in the first rank in a dangerous battle.
— Hans Delbrück, Warfare in Antiquity, pp. 53-55