For more than two centuries, the Achaemenid Empire of Persia ruled the Mediterranean world. One of history’s first true superpowers, the Persian Empire stretched from the borders of India down through Egypt and up to the northern borders of Greece. But Persia’s rule as a dominant empire would finally be brought to an end by a brilliant military and political strategist, Alexander the Great.

Alexander III was born in 356 B.C. in the small Kingdom of Macedonia. Tutored in his youth by Aristotle and trained for battle by his father, Philip II, Alexander the Great grew to become a powerful imperialist. His undermanned defeat of the Persian King Darius III at the Battle of Gaugamela is seen as one of the decisive turning points of human history, unseating the Persians as the greatest power in the ancient world and spreading Hellenistic culture across a vast new empire.

Alexander owed a tremendous debt to his father for leaving him a world-class army led by experienced and loyal generals. But it was Alexander’s genius as a leader and battlefield strategist that secured his victory against an imposing adversary deep in enemy territory.

The Macedonians weren’t always a force to be reckoned with. The historic centers of Greek power were the city-states of Athens, Sparta and Thebes to the south, whose leaders regarded the Macedonians as barbarians. It was Alexander’s father, Philip, who single-handedly transformed the Macedonian army into one of the most feared fighting machines in the ancient world.

Philip reorganized all of Macedonian society around a professional army and raised elite fighting forces of infantry, cavalry, javelin throwers and archers. Aristocratic young men would start their military training at seven years old and graduate to officers at 18. The highest positions were in the Royal Companion Cavalry, the king’s own personal squadron, and in the Royal Hypaspists, an elite 500-man infantry unit that surrounded the king in battle.

Weaponry also got an upgrade under Philip. Gone was the shorter “dory” or Greek wooden spear (7 feet long), and in its place was the much longer sarissa, an 18- to 22-foot hunting spear with an iron tip that could puncture heavy armor and impale charging cavalry horses.

Backed by his shiny new army, Philip marched south in 338 B.C. and defeated an all-star alliance of Athens and Thebes at the Battle of Chaeronea. The battle served as a coming-out party for 18-year-old Alexander, who bravely led the Macedonian cavalry charge that broke through the Athenian ranks and secured victory for the upstart kingdom.

With the Greek mainland subdued under Macedonian rule, Philip turned his well-oiled army East toward the Persian Empire, a far greater prize. But soon after crossing the Hellespont into Persian territory, Philip was assassinated, making young Alexander the new king and commander-in-chief of the Macedonian forces.

“As soon as Alexander came to the throne, he openly stated that he would carry on his father’s plans,” says Graham Wrightson, a history professor at South Dakota State University and author of Combined Arms Warfare in Ancient Greece. But before Alexander could push into Persia, he had to take care of business back home.

The Greek city-states of Athens and Thebes weren’t thrilled to be under the thumb of “barbarian” kings, particularly since it infringed on their democratic ideals. Immediately after Alexander was made king, Thebes rose up to challenge his authority—a big mistake. Not only did the Macedonian army easily crush the Thebian rebellion, says Wrightson, “but Alexander razed Thebes to the ground and sold the entire city into slavery, except for one house owned by the descendants of his favorite poet.”

Always the savvy strategist, Alexander knew that he couldn’t rule the Greek mainland by fear and brute force alone. So as he turned his attention back to Persia, Alexander framed his campaign against the Achaemenid Empire as a patriotic retaliation for Persia’s failed invasion of the Greek mainland a century earlier. That conflict featured the famous Battle of Thermopylae, where 300 Spartan warriors made a heroic last stand against tens of thousands of Persian invaders.

“Alexander creates a propaganda campaign that the Macedonians are invading Persia on behalf of the Greeks, even though Macedon wasn’t part of Greece and didn’t fight on the side of Greece in the original Greco-Persian wars,” says Wrightson. “He’s invading Persia to punish the Persians retroactively for daring to invade Greece in the first place.”

Whether motivated by Greek pride or the spoils of imperial conquest, Alexander picked up where his father left off and marched into Persia in 334 BC, where his army of 50,000 would be tested against the largest and best-trained fighting force in the known world.

It’s estimated that King Darius III of Persia was in command of a total of 2.5 million soldiers spread across his vast empire. At the heart of the Persian army were the “Immortals,” an elite regiment of 10,000 infantrymen whose numbers never changed. When a man was killed, another rose to take his place. The Persian cavalry and archers were also legendary, as were the scythe chariots which cut down enemy infantry with their razor-sharp wheel hubs.

But there were also signs that the Persian Empire was already in decline. After suffering humiliating back-to-back defeats in Greece in the 5th century B.C., Persia stopped expanding. In the century leading up to Alexander’s reign, Persia was further weakened by a civil war and other internal rebellions. Darius still commanded a massive army, but Persia was receding on the world stage while Macedon had the momentum of an ascendant military superpower.

After quickly dispatching a small regional army near the town of Granicus, Alexander had his first real test against Darius and his Persian Royal Army near the coastal city of Issus. Darius’ strategy was to cut off Alexander’s supply lines from behind and force the Macedonian troops to turn around and face off. But Darius botched the location of the battle, which ended up being a narrow strip of land between a ridge and the sea that neutralized his numbers advantage.

At Issus, Alexander debuted the battle strategy that would assure him victory after victory during his remarkable reign of conquest. Knowing he would be outmatched in manpower, Alexander relied on speed and distraction. He would draw enemy troops toward one flank, then wait for a momentary gap to open up in the center of the enemy lines for a head-first cavalry charge.

Just as he did with his father at Chaeronea, Alexander personally led the Macedonian cavalry charge at Issus, which cut right to the heart of the Persian defenses, just as planned. A stunned Darius reportedly hopped on his horse and fled, with the rest of his army close behind.

The two armies wouldn’t meet again for another two years. In the interim, Darius regrouped and called in reinforcements from the East, while Alexander marched his army South into Egypt. When Alexander returned to Persia from his Egyptian conquests, Darius tried to delay the inevitable clash as long as possible, eventually deciding that if there was going to be a rematch, it would be on Daruis’ terms.

Darius and his generals chose a battle site near the town of Gaugamela. It was a wide, flat valley that, unlike Issus, would allow the Persians to take full advantage of their lopsided numbers, an estimated 250,000 Persian troops facing off against Alexander’s 50,000.

“Darius even flattened the ground so that his scythe chariots could charge at the Macedonians,” says Wrightson.

But Alexander will not be outplayed. He camped the Macedonian army in the hills above the battle site to fuel up and rest while he drew up a game plan. The Persians, fearing a night attack, remained in ready formation all night, anxiously awaiting a charge that never came.

At dawn, the Macedonians took the battlefield. True to his strategy, Alexander’s army advanced in a line with the two flanks drawn back like a bow. Then he ordered the entire Macedonian line to march quickly to the right.

Darius, fearing he was about to be overlapped on his left side, sent in 5,000 of his best cavalry. Alexander counter-struck with a regiment of 1,500 mercenaries tasked with holding the right-hand position. Darius grew frustrated with the lack of progress, so he sent in another 10,000 cavalry, almost his entire left flank. Alexander responded with what’s known as his “pawn sacrifice” of several thousand troops destined to die as a set up for the final move.

At this point, Darius ordered a full-frontal charge on the rest of the Macedonian army, but it took time for his orders to reach his left flank. This created just enough slack in the Persian line for Alexander to strike.

“Just as Darius begins the charge, the Macedonians launch a devastating cavalry attack that goes right into the gap cunningly created by Alexander’s tactics,” says Wrightson.

As Alexander and his elite Royal Companion Cavalry raced into the heart of the Persian defenses, they were momentarily surrounded by the enemy, but the experienced Macedonian sarissa regiments fought their way through. According to legend, Alexander killed Darius’ chariot driver and almost captured the Persian king before he fled once again on horseback.

Days later, with Alexander’s cavalry in hot pursuit, Darius was killed by his own cousin, who delivered the fallen king’s head to Alexander as a tribute. Appalled by the treasonous act, Alexander had the man tortured and executed before declaring himself the undisputed king of Macedonia, Greece, and now Persia.

The reign of Alexander the Great was short-lived. After subduing all of the Persian Empire, his army marched east and got as far as India before turning back home to Macedon. But he never made it home. At just 32 years old, Alexander died in Persia in the palace of Nebuchadnezzar II in Babylon from a sudden and mysterious illness.