Alexander the Great and his father, Philip V of Macedon, were the first "modern" leaders to recognize the geopolitical implications of their activities. In this recognition lay the key to their unqualified success as conquerors. By the reign of Philip V, the alliances between various Greek city-states and his kingdom were complex. As a member of the Sacred League, Philip had warred on behalf of various city-state alliances over the years. Upon the invitation of Thessaly, another "semi-barbarian" country valued by the Greeks for their cavalry, Philip of Macedon joined his realm with theirs - and was suddenly an independent power in the chess game of peninsular politics. Philip was uneducated himself in Greek standards of learning, but he was not unlearned in their ways. As a teenager he was kept as diplomatic hostage in Thebes for several years: Philip saw their infantry at close range, and, being in diplomatic circles, observed the strengths and weaknesses of the Greeks' ever-shifting "federal" alliance systems. If one is to understand Alexander's subsequent military success leading Macedonian armies to conquest, one must study the lessons of his best teacher, his father.
As a matter of tradition, the usual Greek army was one drawn up for a specific war. The Spartans of old made their citizens a permanent warrior class, living off a permanent slave caste - an arrangement long since debased by Philip's time through internal betrayals of the Spartan meritocratic tradition. Most of the other city-states, like Athens, depended upon cadres of hoplites: heavily-armed infantry of citizen-volunteers. As citizenry was a status to be valued (universal suffrage not the status quo), hoplites were originally troops of highly-motivated, patriotic individuals - as they proved during the Battle of Marathon (490 BC) and elsewhere a century and more earlier.
Generations of constant war had bankrupt the city-states. Citizen morale...