Alexander's diagonal advance, the confusion which it caused, the break in the Persian line, and its prompt occupation by some of the best cavalry and a portion of the phalanx, are the turning-points of the engagement.

All the rest followed as a matter of course.

Far too much importance has been assigned to Darius's flight, which was the effect rather than the cause of victory.

When the centre of an Asiatic army is so deeply penetrated that the person of the monarch is exposed and his near attendants begin to fall, the battle is won.

Darius did not--indeed he could not--"set the example of flight." Hemmed in by vast masses of troops, it was not until their falling away from him on his left flank at once exposed him to the enemy and gave him room to escape, that he could extricate himself from the melee.
No doubt it would have been nobler, finer, more heroic, had the Persian monarch, seeing that all was lost, and that the Empire of the Persians was over, resolved not to outlive the independence of his country.