On the throne
Alexander was born in 356 BC in Macedonia, the area around present day Thessaloniki in northern Greece. Though the Macedonians might have considered themselves part of the Greek cultural world, the other Greeks might have viewed them as half-barbarians. Alexander's father, King Philip, was an energetic ruler who had started a systematic policy of expanding his kingdom. Philip's main conquest was that of the Greek mainland, after his victory at Chaeronea in 338 BC. Alexander, still in his teens, commanded the Macedonian cavalry during this battle.
In 336 BC King Philip was killed and Alexander ascended to the throne of Macedonia. Within the next twelve years Alexander conquered almost the entire known world of his era. Though Alexander made use of the well-oiled army created by his father, he pushed the limits of Macedonian & Greek power to levels King Philip could not have dreamed of.
Persian foe
Alexander's main opponent was the Persian King Darius III. The Persian kingdom was an empire of epic proportions - stretching from Egypt and the Mediterranean into India and central Asia - which had dominated the ancient world for over two centuries. Most sources claim King Darius commanded incredibly huge armies. One million Persians are said to have taken the field against Alexander in 331 BC.
Alexander defeated Darius during three major engagements. In 334 BC he swept away a Persian defence force, sent by King Darius, at the river Granicus (Turkey). In 333 BC Darius faced the invader in person near the town of Issus (southern Turkey), but suffered a massive defeat. Alexander's brilliant final victory, at Gaugamela (Iraq) in 331 BC, irrevocably changed the course of history.
Edges of the earth
The capitals of Persia were now in Alexander's hands and he gained possession of the unbelievable gold reserves of the former Great Kings. However, Alexander and his army marched ever further eastward, battling nomadic warriors and rebels on the north-eastern fringes of the known world. In 326 BC he defeated King Porus at the river Hydaspes (India). But after this major, though difficult victory his troops refused to continue their conquests and Alexander finally ordered the return to his new capital Babylon.
During the 'retreat' many of his men perished in disastrous desert marches. Those who returned safely with Alexander had covered over 20,000 miles within a period of roughly ten years. In June 323 BC, back in Babylon, Alexander fell ill and died after ten days of high fever. Though his first wife Roxane was pregnant with her first son, Alexander the Great left no heir.
Leadership
So, why does he deserve the title "the Great"? Alexander was surely not the first person in history who got this title. The Persian King Cyrus the Great and the Egyptian Pharaoh Ramses the Great went before him. But it is recorded that even in Antiquity the Roman emperors already knew Alexander as "the Great".
The first clue is Alexander's leadership. Military experts still consider him one of the most outstanding commanders ever. Arguably, there is no one else in history who could inspire and motivate his men like Alexander did. Many explanations have been suggested: he suffered the same wounds as his soldiers, he payed attention to every single man in the army and he always led the attack in person. (Actually, he was the last great commander in history to take this personal risk.)
But apart from all that there must have been a deciding factor that we can only marvel about: charisma. Alexander was the only individual whose personal authority could hold his huge empire together. After his death it almost immediately fell apart into competing kingdoms. In 332 BC, in Egypt, the famous oracle of Siwa allegedly confirmed that Alexander had divine origins and that the god Zeus (Ammon) was his true father. We do not know how Alexander himself thought about his divinity, but it surely helped him to boost the myth around his person.
Changing world
Alexander ranks among figures like Jesus Christ and Napoleon in the category "individuals who shaped the world as we know it". Before Alexander world civilisation had been dominated by eastern cultures - Persians, Egyptians, Babylonians. Alexander shifted the spotlight once and for all. From now on the western societies of the Romans and the Greeks would take over the torch.
Alexander had started to mint the gold reserves of the Persian kings and used his resources to continue his conquests and to build new cities and ports. Greek civilisation spread around the known world, improving trade relations and economic activities. The economic system that began to take shape after Alexander's reign remained virtually unchanged until the Industrial Revolution of the late 18th century.
Above that, Alexander had set the limits of what was considered the inhabited earth. It would last until the voyages of the Portugese and Spanish, in the late 15th century, before Europeans were convinced that they had finally explored further than Alexander had done.
God or devil?
Alexander's empire was no rosegarden. Especially after the final defeat of King Darius the court was plagued by controversy and intrigue. Alexander had some of his loyal aides tortured and killed. The justification for these acts is still subject to debate. Especially during the campaign in India, the Macedonians used brutal force to subdue the conquered peoples. Even the sick and elderly, it is written, were butchered.
But also earlier on, during the long siege of Tyre (Lebanon) in 332 BC, Alexander had 2,000 inhabitants mercilessly crucified. In modern Iran he is still known as an evil king - a personification of the devil if you like - who did his very best to destroy the respectable old Persian culture and religion.
On the other hand some western scholars have presented Alexander as a visionary who believed in the peaceful co-existance of different nations and races within his empire. They refer - for example - to mass-weddings ordered by Alexander to reconciliate Greeks and Persians. In the Middle Ages his figure had evolved into a legendary hero, the quintessential example of chivalry and worldly power. In our present time, as he had a lifelong relationship with his comrade Hephaistion, he has been portrayed as a "gay hero".
It is still very difficult to view his personality without being biased one way or the other.