Alexander, King of Macedonia, began his military campaigns in 334 BC against the Persian Empire. His leadership and strategic brilliance led to the creation of one of the largest empires ever known. The video provides a brief historical context of Ancient Greece, highlighting the city-states of Athens and Sparta, their conflict with the Persian Empire, and the rise of Macedonia under King Philip II, Alexander's father. Philip's military reforms, including the sarissa, allowed him to unite Greece, setting the stage for Alexander's invasion of Persia.

Alexander's army, approximately 40,000 strong, consisted of Macedonian phalangites with 18-foot sarissas, elite hypaspists, Greek allied hoplites, and various skirmishers. The Companion Cavalry, led by Alexander himself, formed the shock troops. The Persian satraps, ignoring Memnon of Rhodes' advice to use a 'scorched earth' strategy, decided to confront Alexander at the River Granicus. Despite being slightly outnumbered, Alexander ordered an immediate assault, leading his right wing across the river. The battle was fierce, with Alexander fighting on the front lines. The speed and shock of his attack, coupled with the arrival of the Macedonian phalanx, routed the Persian cavalry. The Persian Greek mercenaries, unable to join the battle, were massacred, marking a major victory for Alexander.

Following the Battle of Granicus, Alexander advanced into Asia Minor. He neutralized Persian naval power by capturing coastal cities like Miletus and Halicarnassus. At Gordium, he famously 'unpicked' the Gordian Knot. Memnon of Rhodes briefly launched a naval offensive in the Aegean, but his sudden death halted the Persian efforts. In 333 BC, Alexander advanced into Cilicia, where he was met by the main Persian army, led by King Darius III, near Issus. Darius, with a numerical advantage, aimed to trap Alexander in a narrow coastal plain. However, the terrain prevented Darius from fully utilizing his superior numbers. Alexander led a daring charge against the Persian center, causing Darius to flee the battlefield. The Macedonian phalanx, initially in trouble, regrouped, and the Persian army was routed, resulting in a stunning victory for Alexander.

After Issus, Alexander subdued the western territories of the Persian Empire. He besieged and captured Tyre and Gaza, and then advanced into Egypt. The Persian governor surrendered Egypt to Alexander, who was welcomed by the priests as a liberator and crowned Pharaoh in Memphis. He founded the city of Alexandria and consulted the oracle of Siwah, where he was hailed as the son of Amun. Meanwhile, King Agis of Sparta, with Persian support, revolted against Macedonian rule in Greece. Alexander's commander Antipater crushed the Spartan army at Megalopolis, securing Alexander's base in Greece. Rejecting Darius's peace offer which included gold, his daughter in marriage, and half the empire, Alexander continued his march east, determined to conquer the entire Persian Empire.

In 331 BC, Alexander confronted Darius III's vast army at Gaugamela. Darius had chosen an open plain to leverage his numerical superiority, estimated between 50,000 and 80,000 men, including chariots and war elephants. Alexander, though outnumbered, deployed his army in his characteristic formation. He executed a feigned flank maneuver which drew Persian cavalry away from the center, then launched a decisive charge with his Companion Cavalry and phalanx directly at the weakened Persian center. This bold move caused Darius to flee once again, leading to the collapse of the Persian army. Despite initial struggles on his left flank, Alexander achieved a complete victory. The road to Babylon and Susa, the major Persian capitals, was now open.

After Gaugamela, Alexander entered Babylon and Susa in triumph, taking his place on the Persian royal throne. He ordered the ceremonial capital of Persepolis to be pillaged and burned as retribution for Persia's past invasion of Greece. Darius fled east, but was murdered by one of his own governors, Bessus, who then proclaimed himself the new ruler. Alexander sent Darius's body to be buried in the royal tombs and then focused on organizing his new empire, appointing viceroys and keeping some loyal Persians in their posts. He then resumed his relentless march east to capture and punish Bessus, subjugate the eastern provinces, and reach the 'edge of the world'.

Alexander pursued Bessus into Aria, modern Afghanistan, crushing a rebellion and founding Alexandria Ariana (Herat). Along the way, he executed Philotas and Parmenion on suspicion of treason, highlighting the brutal politics within his court. He founded Alexandria Arachosia (Kandahar) and eventually captured Bessus, sending him back to Persia for execution. Alexander pushed into modern Tajikistan, fighting Sogdian tribes and founding Alexandria-Eschate, 'the Furthest,' on the Jaxartes River. He decisively defeated Scythian nomads, but the ongoing guerilla warfare in Bactria and Sogdia proved challenging. Many Macedonian troops grew weary of endless conquest. Alexander's adoption of Persian customs and rituals, like proskynesis, caused discontent among his men, leading to incidents like the murder of Cleitus the Black and the uncovering of an assassination plot by royal pages.

Alexander continued his eastern expansion, crossing the Hindu Kush mountains into what is now Pakistan and India. He fought and won against various tribes, including the Aspasii and Assaceni. He formed an alliance with the ruler of Taxila and faced King Porus at the Battle of the Hydaspes, a costly but decisive victory despite Porus's war elephants. Alexander intended to push further into India, but at the River Hyphasis, his army mutinied, refusing to go further after years of continuous campaigning. Forced to turn back, Alexander led his army down the rivers of the Punjab to the sea. During this journey, he was severely wounded during the assault on the Mahlian capital. Part of his army sailed back to Persia under Nearchus, while Alexander led the rest through the Gedrosian desert, an arduous march resulting in significant losses.

Upon his return to Persia, Alexander executed corrupt governors and orchestrated a mass marriage between Macedonian officers and Persian noblewomen at Susa to foster unity. He paid his soldiers' debts and ordered the training of 30,000 Persian youths in Macedonian warfare. At Opis, a Macedonian mutiny erupted over Alexander's perceived preference for Persian ways, which he quelled with a speech. His closest friend, Hephaestion, died of fever, causing Alexander immense grief. Alexander continued to plan new campaigns but suddenly fell ill and died in Babylon at the age of 32 in 323 BC. The cause of his death remains debated. Undefeated in battle, Alexander's empire, though vast, was unstable and quickly fragmented as his generals fought over control in the Wars of the Successors. His legacy includes the spread of Greek ideas during the Hellenistic Age, influencing art, science, and governance across his former empire.