Alexander The Great was one of the most successful kings of Macedon. He conquered Persia and Greece and set up one of the largest empires in the ancient world.
Alexander The Great became king at 20, when his father was murdered by one of his bodyguards. The future king was tutored by Aristotle. After he became king, Alexander conquered Greece and then had an even better idea. Why stop in Greece if he could conquer Egypt too? And why stop in Egypt if he could also have Persia?
After Alexander amassed a huge army, he conquered Egypt, and legend tells that he founded the city of Alexandria, but that might have been constructed after his death. He then attacked Persia and came out on top, now having a large chunk of its territory. His campaign ended when his troops, exhausted and starved, would not follow him any longer and he had to go back South.
After the Persian campaign, Alexander went back to Babylon and while he was there, he caught a fever while in a mosquito-infested swampland. He died of the fever, ordering his troops to the very end.
I think the secret of Alexander the Great's success was that he cared about his army, he fought in the front lines with them and shared the spoils fairly.