When Alexander the Great conquered the world — the Persian Empire, that is — he brought his father’s not-quite-Greek army:
The Macedonian army was an organizational improvement on the Greek hoplite army. The Greeks had developed the practice of fighting in solid ranks, forming a combat block of shields, armor, and spears. The whole aim of battle was to keep one’s troops together in a rectangular mass; with their heavy armor, they could not be hurt by arrows, stones or javelins — a Roman version was called a Tortoise because it was impervious to anything.
The Greek phalanx, developed in the 600s and 500s BC, was a huge shift from the traditional mode of fighting depicted in the Iliad (around 750 BC). The traditional form could be called the hero-berserker style. An army consisted of noisy crowds of soldiers clustered behind their leaders, who didn’t really give orders but led by example. Heroes like Achilles, Hector, and Ajax would work themselves into a frenzy, roaring out onto the battlefield between the armies, sometimes fighting a hero from the other side, but more often going on a rampage through the lesser troops, cowing them into a losing posture and mowing them down with sheer momentum, i.e. emotional domination. This berserker style remained the way “barbarian” armies fought — that is to say, armies that did not have disciplined phalanxes. The hero-berserker could never beat a Greek or Roman phalanx that stood its ground; the Greeks were always victorious over the barbarians to the north and east of them, and so were the Romans over their respective hinterlands.
On the other hand, when one Greek phalanx met another, the result was a shoving match. Unless one side broke ranks and ran away, few soldiers were killed. Most battles were stalemates, and city-states could avoid combat if they wished, sheltering behind their walls. The main purpose of cities all over the ancient Middle East, many of them just fortified towns, were these defensive walls, impervious to berserkers. Phalanxes only fought by arrangement, when both sides assembled on chosen ground for a set-piece battle.
The main weakness of the hoplite phalanx was that it was slow-moving. Hoplites were heavy troops, quite literally from the weight of armor they carried. An enemy that hit and ran away could harass a Greek phalanx but would be beaten if it stayed to fight head-to-head. This was brought home to the Greeks when Xenophon returned from a campaign in Persia during 401-399 BC, writing up their experiences in his famous Expedition of the Ten Thousand. A contender for the Persian throne had hired them as mercenaries; but once they reached the Mesopotamian heartland, the Persian leader was killed in battle, and the Ten Thousand had to fight their way back, first against the Persian army and then against primitive hill tribes on their path to the Black Sea. The Persians troops were somewhere between the berserker style and the disciplined Greeks. They relied on large masses to impress their enemy into submission; typically these were grouped by ethnicity, each with their own type of weapons. Among these weapons of terror were rows of chariots with scythes attached to their axles; sometimes there were war-elephants. Troops recruited from tribal regions were used on the flanks, as clusters of stone-slingers, archers, and javelin-throwers; these were light troops, without armor since they fought from a distance. The Persian armies that Alexander fought had the same shape.
None of these troops could beat a disciplined phalanx that held its ground; the chariots could get close only if they ran onto the phalanx’s spears, which horses are unwilling to do; elephants, too, are hard to control and shy away from spears. The Greeks soon recognized they could beat armies of almost any size if they stuck together. A bigger problem was that enemy light troops, and attacks by tribal forces with arrows and slings, could be repelled by their armor and discipline, but hoplites were too heavy to chase them down and keep them from repeating the attack.
The solution was to add specialized units around the phalanx; hiring their own barbarian archers and slingers, and adding cavalry, mainly for the purpose of finishing off the enemy when they are running away. But in the Greek homeland, most battles were simply phalanx-on-phalanx; in the democratic city-states, this was as much a display of egalitarian citizenship as a military formation.
Philip’s Macedonian army, which he put together between 360 and 336 BC, incorporated all the most advanced improvements. Most importantly, he added heavy cavalry, operating on both flanks with the phalanx in the center. Philip’s cavalry were not just for chasing-down after the enemy broke ranks, but for breaking the enemy formation itself. Philip was one of the first to perfect a combined-arms battle tactic: the phalanx would engage and stymie the enemy’s massed formation, whereupon the cavalry would break it open on the flanks or rear.
This was one of the advantages of Macedonia’s marchland location; having only recently transitioned from tribal pastoralists to settled agriculture, it could combine military styles. Philip’s phalanx was recruited from the peasant farmers, his cavalry from the aristocracy, used to spending their time riding and hunting. Philip’s — and thus Alexander’s — cavalry were called the Companions; they were the elite, the carousing drinking-buddies of their leader. The Companion cavalry, usually on the right wing of battle, was complemented by another cavalry on the left wing, recruited from the Thessalian plains people, but commanded by Macedonian officers.
In addition to improving on the best-of-the-barbarians, Philip also borrowed from the most scientifically advanced Greeks, the colonies in Sicily, for techniques of attacking fortresses. These included catapults and engines, underground mining (to undermine walls), siege ladders and protected roofing to cover the de-construction engineers as they worked on the fortress.