In 336 B.C. a 20 year-old Macedonian from the royal line of a nation numbering not more than 1.5 million people, assumed the kingship of the Greek world from his assassinated father. Alexander had been educated by one of history's greatest teachers, Aristotle. Along with the throne, his father Philip bequeathed him the finest fighting force the world had yet seen. Philip himself had built the Macedonian army, organized and equipped in a highly flexible combined arms manner that had easily crushed the other monolithic phalanxes of the Greek city states and their colonies, as well as irregular formations of barbarian tribes to the far north of Macedon.
Immediately upon the young Alexander's accession to the dominating Macedonian throne, the Greek city states and the barbarians whom his father had subdued tested his mettle and attempted to throw off Macedonian rule. Alexander first solidified his authority as leader of the Hellenic League (alliance of all the Greek states, save Sparta) established by his father by marching on Athens at the head of a Macedonian army and rapidly crushing, by a demonstration of strength alone, the opposition. That done, he turned north and put down the restive barbarian tribes.
While campaigning in the north, Alexander received word that Thebes and Athens, at the instigation of Darius II of Persia, had revolted against Macedonian rule. Alexander immediately marched on Thebes and all but destroyed the city to subdue it. Athens got the message and surrendered peacefully to Alexander's rule, for which the young king returned kindness and no retribution for their errant behavior.
With the Greek world solidly behind him, and two years of successful campaigning against disparate enemy formations under his belt, Alexander crossed the Hellespont into Persian dominated Asia (what is today, Turkey), not just to accomplish his father's dream of punishing the world power to the East for their repeated transgressions against the Greeks, but with his own plan of conquering Persian, AND the rest of the known world. Alexander first defeated a mixed army of Persians and Greek mercenaries at the Battle of the Granicus and liberated the Greek coastal cities of Asia. Lacking a navy with which to maintain his sea lines of communication with his home base back across the Hellespont, Alexander next sought to negate the Persian navy's ability to interdict him, by sweeping down the Mediterranean coastline and capturing all of the ports from which the Persian navy operated.
A third of the way through this coastal campaign, as Alexander was marching southward into modern-day Syria, the Persian king had attempted to cut off his lines of communication back to Macedon by placing a 100,000 man army to Alexander's rear. Alexander had turned around with 30,000 men, and, despite being outnumbered three to one against an enemy well fortified on the other side of the Pinarus River, routed and destroyed half of the Persian army with the loss of less than 500 of his own men. Following this victory, remembered as the Battle of Issus, Alexander completed his anti-Persian Navy campaign by seizing the island port of Tyre (by means of the herculean engineering feat of building a causeway out to the city), the city of Gaza (by means of another great engineering feat: the construction of a 250 foot high mound a quarter mile in circumference at the base from which the Greeks bombarded the defenders with siege engines), and the occupation of Egypt where the young conqueror established the first of many Alexandrias.
Having effectively destroyed the greatest navy in the world at the time, by way of brilliant stratagem, Alexander now turned his attention to the mighty Persian Army assembling in Mesopotamia. With his trademark rapid march, Alexander sped from Egypt back to Tyre, turned east and crossed the Euphrates and Tigris Rivers. The Persians, numbering about 200,000, were drawn up in battle array on the Plain of Gaugemela waiting for Alexander. Outnumbered nearly four to one, the Greeks nonetheless attacked straightaway and Alexander's tactical genius shown clearly in his adaptation of a battlefield formation that created, then exploited, a gap in the center of the massive Persian line. The Persian king, Darius, was suddenly in a relatively exposed position and he beat a hasty retreat. His grand army disintegrated with his ignominious retirement from the battlefield, and, after being pursued by Alexander for the next eleven days, Darius was assassinated by a group of his Persian nobles, leaving the young Macedonian strategic, operational, and tactical genius the ruler of Persia--a feat the most ambitious Greek leader before him could never have dreamed attempting.
No warrior king had ever before nor has ever since accomplished so much conquest at any age and level of experience, let alone Alexander's tender age of 25. He was clearly the world's most brilliant battlefield leader and destined to cruise to world domination, including the lands that were later to encompass the western Roman Empire--what is now Italy, Mediterranean North Africa, Spain, France, and the British Isles. He could count on vigor that would keep him in the fight for at least another 25 years. He had consolidated control of the Greek world, occupied Egypt, and conquered the Persian Empire in less than five years--one could scarcely fathom what he should accomplish given five more like periods of time...
But, then he marched into Afghanistan.