Why was Alexander the Great able to conquer Persian empire?

Answer 1

Alexander inherited the project of conquering the Persian Empire from his father Philip II who had planned the conquest, but was assassinated on the eve of his departure for Asia. He followed Phillip's plan:

He kept up his fighting strength up by enlisting a flow of Greek and other mercenaries to supplement his Macedonian army.

After the first victory in Asia Minor at Granicus he murdered the Greek mercenaries in the Persian army to warn other Greeks from serving with them, so depriving the Persians of armoured infantry who could stand up to his own.

He conquered the Persian states around the south east Mediterranean Sea, depriving Persia of a naval force which could threaten Macedonia/Greece in his rear.

After the second defeat of Persia at Issus, he captured the Persian treasury, and so was able to afford to pay and sustain his army.

He moved with the seasons, at harvest time, so he was able to feed his armies.

He finally defeated and dispersed the Persian army at Gaugamela which ended combined Persian resistance, and installed provincial governors, leaving him to progressively mop up separate pockets of resistance.

Answer 2

Alexander inherited the project of conquering the Persian Empire from his father Philip II who had planned the conquest, but was assassinated on the eve of his departure for Asia. He followed Phillip's plan:

He kept up his fighting strength up by enlisting a flow of Greek and other mercenaries to supplement his Macedonian army.

After the first victory in Asia Minor at Granicus he murdered the Greek mercenaries in the Persian army to warn other Greeks from serving with them, so depriving the Persians of armoured infantry who could stand up to his own.

He conquered the Persian states around the south east Mediterranean Sea, depriving Persia of a naval force which could threaten Macedonia/Greece in his rear.

After the second defeat of Persia at Issus, he captured the Persian treasury, and so was able to afford to pay and sustain his army.

He moved with the seasons, at harvest time, so he was able to feed his armies.

He finally defeated and dispersed the Persian army at Gaugamela which ended combined Persian resistance, and installed provincial governors, leaving him to progressively mop up separate pockets of resistance.