Although victories such as Marathon (490 BCE) had stopped the Persian expansion into mainland Greece, the Greek cities along the Ionian coast of the Aegean remained firmly under the control of the Persian Empire. As a result, the cry for a war of revenge against the Persian Empire remained a strong focus in the art and politics of the ancient Greek world.
Parochial hatred and constant petty infighting prevented the Greek states from initiating this war of revenge. Athens lost its empire to Sparta in the Peloponnesian War (431-404). Spartan domination (404-371) was mismanaged, Thebes defeated Sparta and became the dominant state (371-362) but was unable to expand, leading to the reemergence of the Athenian Empire in 362.
Thus, in spite of this century-long call for revenge, none of the major city-states had displayed the cohesiveness, military strength, or diplomacy needed to achieve the complete hegemony over all Greece required to prosecute a successful campaign against the Persian Empire.
In 359 BCE Macedonia was a weak tribal kingdom when the Illyrians invaded Upper Macedonia from the west. King Perdiccas III was defeated and killed in battle leaving much of western Macedonia occupied by the Illyrians. The surviving remnants of the Macedonian army elected his younger brother Philip to be their new king.
Philip slowly and carefully expanded his small kingdom into a military superpower, such that by 338 he had become hegemon of Greece and was preparing to invade the Persian Empire when he was assassinated in 336. It would be left to Philip's son "Alexander the Great" to lead the army created by his father on one of the greatest military campaigns in history.
In the 4th century BCE, the armies of the Greek city-states were made up of a core of disciplined hoplite heavy infantry. They fought in dense phalanx formations forming into battleline for engagements. Hoplites were equipped with a 3-foot diameter hoplite shield, greaves, breastplate, helmet and 10-foot spear.
Peltasts were light infantry that handled light garrison duties and reconnaissance. As an auxiliary force in battle, peltasts skirmished to disrupt and soften enemy formations prior to the main battle engagement involving hoplites. Peltasts were equipped with a crescent shaped pelta shield, helmet, javelins and short spear.
The small numbers of cavalry present were fielded by the wealthy and remained relatively inactive in battle. Cavalry were equipped with greaves, breastplate, helmet and a 12-foot lance. Even when active, cavalry were ineffective against organized infantry formations. This was aggravated by the fact that stirrups, required for stability at the point of impact, had not yet been invented. Thus, cavalry was left to rely on speed and mobility to exploit weakness and disorder in poorly organized or disrupted infantry.
The typical non-Greek army was composed of the king and nobles who formed a squad of heavy cavalry, equipped with greaves, breastplate, helmet and 12-foot lance.
Heavy infantry were armed with a 2-foot shield, helmet and 9-foot spear, and although they formed a massed battleline, lacked the cohesive discipline of hoplites.
Light infantry, including peltasts and other infantry equipped with javelin, sling or bow, handled reconnaissance and skirmishing. The bow was not commonly used in formations of significant size as it required constant upkeep and training.
Armies fielded by the Persian Empire were similar to those of a tribal kingdom except that they contained large contingents of heavy cavalry and a variable number of chariots. Learning from losses sustained in previous wars against the Greeks, a significant contingent of Greek mercenary hoplites were hired to backbone their main battleline.
Chariots had a terrifying effect on poorly organized infantry in flat open spaces, but were relatively ineffective in rough terrain or against disciplined heavy infantry. Thus, chariots were generally not seen in the Aegean theatre.
The Immortals, with shield, armour, bow and spear, were held in reserve, forming the Great King's elite infantry bodyguard.
When Philip took power the Macedonian army was structured along the lines of a typical tribal kingdom. Philip had a royal squadron of Companion cavalry, each armed with a lance. The infantry was comprised of a poorly organized mass that fought in irregular formation with spears.
As Macedonia was poor, with no middle class able to purchase hoplite armor, Philip armed his light infantry with a 2-foot diameter shield, greaves, helmet and sarissa (a 15-foot, two handed spear), and trained them to fight in an extra-dense formation. This lightly armed but highly disciplined "Macedonian phalanx" combined the cohesive melee strength of dense heavy infantry formations with some of the speed and maneuverability of light infantry.
Philip trained his Companion cavalry to manoeuvre in a face-forward wedge formation that disguised directional change and allowed a flexible, rapid response.
To augment his phalanx and Companions and round out this formidable combined arms field army, Philip added auxiliaries, trained to scout, forage, skirmish and pursue, and a siege train to provide artillery support.
In battle, the resultant advantages gained in initiative and flexibility allowed the Macedonian phalanx to engage and pin the enemy battleline while pressing to create a breach. When a breach was obtained, the Companions would quickly move into position and rush the gap, attacking the enemy infantry in flank and rear. If the breach was fully exploited before it could be plugged, the entire enemy force was disrupted with the battle quickly degenerating into a rout.
Thus, a properly exploited breach became the key to victory. In effect, the phalanx was the anvil while the Companions provided the hammer blow to break the enemy army. In the ensuing pursuit, if a walled city were not close enough to retreat into, then Philip's aggressive use of "extended pursuit" would devastate the enemy forces.
Prior to Philip, sieges were fought out in a manner similar to the legendary siege of Troy. A city would be besieged (cut off from supply) and forced to capitulate after a period of imposed hardship. If the garrison was capable, sorties and smuggling could maintain a trickle of supply, stretching the siege out for years, with victory (if achievable) more often the result of betrayal (a dissident faction would simply open the gates; the metaphorical twist on the Trojan Horse) than through simple starvation or any feat of arms. If unsuccessful, the besieger was eventually forced to withdraw in the face of growing physical, financial and diplomatic exhaustion.
By advancing the development of torsion catapults, Philip assembled an innovative and technologically advanced siege train. These improvements in siege-craft allowed diplomatic overtures to be increasingly backed by the solid physical threat of a conclusive siege.
For the actual siege, Philip would simultaneously encourage discord among the defenders through bribery, subterfuge and the offer of leniency in exchange for early capitulation, all the while using an aggressive multi-pronged military approach. Bolt-firing catapults helped reduce the effectiveness of defenders by keeping them off the walls allowing more effective mining and the greater freedom of movement required to mount numerous focused assaults.
With this coordination of diplomacy, steady harassment and frequent assault, success was usually achieved within three to six months rather than the normal three to six years. If victory was not achieved quickly, financial strain and call for forces elsewhere would usually result in Philip's withdrawal.
In addition to his other skills, Philip displayed a genius for diplomacy, cultivating friendships with touring theatre groups and philosophers (e.g. hiring his boyhood friend Aristotle to tutor Alexander), and through his strategic control of the gold mines of the Mt. Pangaeus region, lavishing money on friend and artisan alike.
In exchange for this patronage, Philip was provided with current information, both political and military, concerning other states of the Greek world, and was often able to use this espionage to soften-up targeted cities, even shifting some to his will without the need for direct military action.
Philip's diplomatic and military activities were sustained by wealth generation. Mining, trade, tribute and booty contributed to Philip's disposable income. As he continuously spent whatever wealth came in, Philip had a keen appreciation of the need to maintain a steady cash flow.