King Philip II of Macedon built a powerful army and conquered Greece in the 4th century BC. He introduced innovations like the Macedonian phalanx armed with long pikes. His son, Alexander the Great, further strengthened the army and used its combined arms tactics to invade and conquer the vast Persian Empire between 334-331 BC. At the pivotal Battle of Gaugamela, Alexander defeated the Persian king Darius III, bringing an end to the Persian Empire and allowing Alexander to expand his empire across Western Asia and into India. However, Alexander's troops grew exhausted by the campaign and he died of illness in 323 BC at the age of 32, leaving his generals to divide up his empire.