Quick questions on Darius III and the conquest by Alexander (HSC Ancient History Section IV)

Darius III came to a throne already destabilised from within. The powerful eunuch and chiliarch (vizier) Bagoas had poisoned Artaxerxes III (Ochus) in 338 BC and then his short-lived successor Artaxerxes IV (Arses) in 336 BC, wiping out the direct line. Bagoas raised to the throne a more distant Achaemenid, Artashata, a satrapal courtier said to have won distinction in single combat against a Cadusian champion; he took the throne name Darius III. When Bagoas judged the new king too independent and prepared to poison him as well, Darius forced the kingmaker to drink his own cup.

The first battle was fought by the western satraps (among them Arsites and Spithridates) together with the Greek mercenary commander Memnon of Rhodes, at the River Granicus in north-western Asia Minor. Memnon reportedly urged a scorched-earth withdrawal to starve Alexander out, but the satraps, unwilling to devastate their own provinces and confident in their cavalry, chose to fight at the riverbank. Alexander forced the crossing, several satraps were killed in the melee, and the Greek mercenaries in Persian service were surrounded and destroyed.

Darius now took command in person. In November 333 BC he made a bold move, marching behind Alexander to cut his line of communication, and forced battle on the narrow coastal plain by the River Pinarus. The stroke was clever, but the confined ground cancelled Persia's numerical advantage (the ancient figures, hundreds of thousands, are inflations).

For the decisive encounter on 1 October 331 BC, Darius chose his ground carefully, an open plain near Arbela in northern Mesopotamia, cleared so that his cavalry, scythed chariots and full imperial levy could be used. This was a considered attempt to fight on Persian terms. Yet Alexander's oblique advance to the right drew the Persian cavalry out of position, a gap opened, and the Companion wedge charged towards Darius, who fled a second time.