When he heard of his son-in-law's great victory he crossed the Euphrates, and advancing through Western Mesopotamia, from which the Persians probably retired, took up his residence at Nisibis, now the chief town of these parts.

It is perhaps true that his object was "to moderate, by his presence and counsels, the pride of Galarius." That prince was bold to rashness, and nourished an excessive ambition.

He is said to have at this time entertained a design of grasping at the conquest of the East, and to have even proposed to himself to reduce the Persian Empire into the form of a Roman province.
But the views of Diocletian were humbler and more prudent.

He held to the opinion of Augustus and Hadrian, that Rome did not need any enlargement of her territory, and that the absorption of the East was especially undesirable.

When he and his son-in-law met and interchanged ideas at Nisibis, the views of the elder ruler naturally prevailed; and it was resolved to offer to the Persians tolerable terms of peace.