Alexander III, acceding at age twenty to the throne of Macedon in 336 BCE, had set his mind on the Persian invasion conceived by his father Philip, having grown up to the idea.

Moreover, he needs the wealth of Persia if he is to maintain the army built by Philip and pay off the five hundred talents he owes.

The exploits of the Ten Thousand, Greek soldiers of fortune, and of Agesilaus of Sparta, in successfully campaigning in Persian territory have revealed the vulnerability of the Persian Empire.

With a good cavalry force, Alexander can expect to defeat any Persian army.