After which victory, Darius being killed, the state remained secure to Alexander, for the above reasons.

And if his successors had been united they would have enjoyed it securely and at their ease, for there were no tumults raised in the kingdom except those they provoked themselves.
But it is impossible to hold with such tranquillity states constituted like that of France.

Hence arose those frequent rebellions against the Romans in Spain, France, and Greece, owing to the many principalities there were in these states, of which, as long as the memory of them endured, the Romans always held an insecure possession; but with the power and long continuance of the empire the memory of them passed away, and the Romans then became secure possessors.

And when fighting afterwards amongst themselves, each one was able to attach to himself his own parts of the country, according to the authority he had assumed there; and the family of the former lord being exterminated, none other than the Romans were acknowledged.
When these things are remembered no one will marvel at the ease with which Alexander held the Empire of Asia, or at the difficulties which others have had to keep an acquisition, such as Pyrrhus and many more; this is not occasioned by the little or abundance of ability in the conqueror, but by the want of uniformity in the subject state..