Philip II was assassinated in 336 BC. just as he was about to lead a combined force of Greeks and Macedonians across the Hellespont to free the Greek states of Asia Minor from the Persians. The Greeks who where allies of Macedonia, were hoping they might repudiate unpopular agreements made with Philip, revolted in Thebes in 335 BC. Hearing of the revolt, Alexander moved his army to Thebes by forced marches within two weeks. The Thebans put up strong resistance, but Alexander breached their walls and overpowered the defenders. As a warning to the other Greeks, Alexander destroyed Thebes, except for their temples. The remaining Greek states hastened to offer ships and men for Alexander's next venture, an attack on the Persians in Asia Minor. In the spring of 334 BC. Alexander crossed the Hellespont, defeating a strong Persian force at the Granicus River, and moved southward along the coast to free the major Greek cities of Aeolis and Ionia.
The following year Alexander moved his forces eastward across ancient Phrygia to Gordius. According to legend, Gordius, founder of Phrygia, tied an intricate knot in the thong connecting the pole of his chariot with the yoke. According to an oracle, the person to untie the knot would rule Asia. Although Alexander tried and failed, he escaped the ill omen by cutting the knot with his swart. Historically, because the Persians failed to defend the Cilician Gates, a pass through the Taurus Mountains, Alexander was able to reach northern Syria. There he encountered a large Persian force commanded by King Darius III, whom he routed in the subsequent Battle of Issus in 333 BC. Darius fled before the battle ended.
During 332 and 331 BC. Alexander founded the Nile city of Alexandria. Meanwhile Darius had asked for peace, offering Alexander all the territory he had won in addition to ten thousand talents and the hand of his daughter. Alexander, however, continued his advance through Syria and access the Euphrates and Tigris into Assyria where he fought Darius again. The Persians again were routed; Darius again fled across the mountains to Media as Alexander entered Babylonia with little resistance. In 330 BC. Alexander forced his way into Persia, occupying its capital, Persepolis. In the spring of that same year, he marched north to Ecbatane against Darius, who was killed by his own officers as he fled to the Persian province of Bacteria, north of the Hindu Kush Mountains.
Alexander was now king of Persia, but he had to secure his title by the conquest of the remaining Persian provinces which lay eastward to the Indus River. Between 330 and 325 BC. Alexander campaigned across modern Afghanistan and Turkestan and eventually penetrating India. When his soldiers refused to proceed further east, Alexander returned to Babylon in 325 BC., having fulfilled as nearly as possible the prophecy of the Gordian knot.
Alexander's tremendous empire was now a major problem. His subjects spoke many different languages, and there where many differed customs. In the old Persian Empire Alexander was an absolute monarch, in Egypt he was worshiped as a god, to the Greek he was merely a commander-in-chief, and in Macedonia he was anything to absolutism, as a matter of fact, divinity was resented. Alexander, in order to unite his Empire, he desired to model his whole government after Persian absolutism. He had planned to create a ruling class by intermarriage of Macedonian and Persian nobles. He himself married foreign, Roxanne of Bacteria and later a Persian princess.
In the midst of his ambitious projects for the future, Alexander was stricken with a fever in 323 BC. and died within a few days, at the age of thirty-three. Although the political entity created by Alexander failed to survive him, he was nevertheless instrumental in creating a uniform economic and cultural world stretching from the Straits of Gibraltar to the Indus River. In the succeeding Hellenistic Age (323-146 BC.) the ancient world came to have a common Graeco-Oriental culture. A single trade area was opened to the merchants of each region and all benefited from the release of Persian bullion. The new trade areas opened we know today as the famous "Silk Routes."
There are many useful websites about Alexander the Great:
Alexander the Great History Project - comprehensive information on everything from his parents to his tactics and weapons.
Alexander the Great Project by John J. Popovic - Very informational site including sections on invasion of Persia and Central Asia.
History of Western Civilization, Boise State University.
In the Footsteps of Alexander the Great With Michael Wood, comprehensive site from PBS.