Alexander III of Macedon (356 BC - 323 BC) was a king of the ancient Greek state of Macedon. Commonly known as Alexander the Great, he lead a Greek expedition to Asia and succeeded in conquering the Persian Achaemenid empire.

Alexander's life and reign are thoroughly discussed in the relevant Wikipedia article, please read it before you ask a question. Additionally you might be interested in reading these related and equally thorough articles:

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for general questions about ancient Greece
for questions specific to the Greek Dark Ages (the Homeric era)
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for questions specific to the Greco-Persian Wars
for questions about the Hellenistic period
for questions about the ancient kingdom of Macedon