In Hegemony: Phillip of Macedon, the Persian empire lies to the east, ready for war and able to be invaded.
Historically, the united Greek forces were called together for this war under Phillip's banner in 337 BCE.
In 332 BCE, Alexander the Great, Phillip's son, visited the Temple of Amun in Egypt. And two hundred years before that, a giant Persian army, 50,000 strong, disappeared while marching on the Temple of Amun.
How things could have changed if Persia had come out stronger, having conquered much more of Egypt, before the Greeks invaded.
Well, that army may have now been found.