Alexander was a brilliant tactician with a well-equipped army and strong desire for conquest.

He was able to unite all the Greek city states and then proceeded towards the common adversary of the Greeks, the Persians. Alexander conquered the whole Persian empire in series of battles which included whole of the West Asia including Egypt.

He didn't conquer the whole world - just the known world (the world that was known to the Greeks at the time). To conquer the entire world at that time would have been impossible.

Alexander used a tactical military called a phalanx which was highly effective against less advanced militaries. It enabled the Greek infantry to wield extremely long spears. Because there were several rows of soldiers, they could control a spear that was to long/heavy for a single soldier.

This gave them a hug advantage over infantries equipped with shorter spears.

It's hard to to battle with the enemy when they can spear you before you can spear them.

It's amazing how that one innovation proved so valuable.

Alexander's army defeated every opponent they faced, and were often outnumbered over two to one.

The spear length advantage was a big part of that.

He used a combination of a well-trained infantry and a highly effective cavalry to outmaneuver his enemies.

He had a strong will to do so and also didn't give a damn about the consequences. He was only a young man when he set out on his quest to conquer the world.

There were no worthy adversaries at the time.

He was more ruthless most. That was a big advantage.

He was known to slaughter thousands and enslave women/children after defeating some of his enemies.

He had good generals. He took over a military largely built by his father, Philip II.

One big part of his success was that he had a great education. As crown prince, he received the finest education in the Macedonian court under his celebrated tutor Aristotle.

Youthful ambition played a critical role. Alexander did not know that conquering the known world was something that could not be done... so he did it. An older man would never have tried.

Alexander’s impressive record was largely due to his smart use of terrain, phalanx and cavalry tactics, bold and adaptive strategy, and the fierce loyalty of his troops.

Great tactics and a brilliant military mind.

Good genes. His father (Philip II) was a brilliant military tactician as well.

His father was Zeus. It helps to have a father that is king of the GOds.

Guts and brains.