In 334 B.C., Alexander left his country with 30,000 infantry and 5,000 cavalry force.
The size of his army continued to grow as he advanced further and further. It was a disciplined and well organised army under efficient generals.
The Macedonian Phalanx represented an advanced method in warfare to enable Alexander to win battle after battle. The Greek hero also took with him a troop of scholars, scientists, historians, linguists, geographers, and architects to observe the new countries and discover the East for the knowledge of the West.
Alexander entered into vast Persian Empire which was then ruled by Emperor Darius III. The armies of Persia could not offer effective resistance to the invaders and were defeated in two great battles. After conquering a large part of Persia, Alexander next proceeded towards Egypt which was, in those days, under the Persian rule. In 332 B.C. the Greek invader conquered the Nile Valley and made it a part of his Greek empire.
Alexander became so proud after conquering that land of the Pyramids and of ancient civilisation that he crowned himself in oriental manner in imitation of the ancient Egyptian Pharaohs. There, too, in one of the mouths of river Nile, he laid the foundation of a new city which became famous as Alexandria. This city was destined to play a great role in future ages as a meeting ground of eastern and western cultures and commerce.
In 331 B.C. Alexander returned to Persia from Egypt to fight against Darius III. At a place named Gaugamela near Arbela the Greek and the Persian armies met. Darius was badly defeated and his army was routed. The battle of Gaugamela or Arbela sealed the fate of the Persian Empire. The defeated Darius III was murdered by one of his treacherous generals. With that, Alexander became the master of the entire Persian Empire.
The conquests of Alexander were already too vast. But, the ambitions of the victorious hero knew no bounds. After Persia, he thought of India, and advanced eastward. With an ever increasing army, and in swift speed, he marched towards the frontiers of India, and, at last, in 327 B.C. he reached the Hindu-Kush with his Greek army.