Alexander the Great became king of Macedon in 336 BC. He succeeded his father, Philip II, who was assassinated while attending a royal wedding at Aigai, the ancient Macedonian capital.
Alexander was about 20 years old when he took the throne. Macedonian nobles and the army proclaimed him king, and he quickly secured his position by suppressing rival claims and rebellions. He also inherited his father's leadership of the League of Corinth, giving him authority over much of Greece.
In 335 BC, Alexander campaigned in the Balkans and destroyed the rebellious city of Thebes. The following year, in 334 BC, he crossed into Asia and began his invasion of the Persian Empire. Thus, 336 BC marks the start of his reign, while 334 BC marks the beginning of his major Asian conquest.
The other dates are tied to important moments in his story: 338 BC was the year of Philip's victory at Chaeronea, and 323 BC was the year Alexander died in Babylon. Those events can make the dates easy to confuse.