Hoplite

The heavy infantryman of the Greek city states. Bronze helmet, breastplate and greaves, a round shield about three feet across carried on the left arm, a thrusting spear, a short sword for when the spear broke.

The shield is the whole logic of the thing. It covers the left half of its bearer and the right half of the man beside him, so that a line holds only as long as every man in it stands. Thucydides notes the consequence: armies drift rightward as they advance, each man edging toward the shelter of his neighbour's shield.

Herodotus gives the Spartans at Thermopylae turning their backs and running as though in flight, then wheeling when the Persians broke ranks to pursue. That manoeuvre needs a line that can stop and reverse on command, which is what the years in the agoge were for.