But empires move, though rulers bid them stand still.
There was no attempt to overwhelm whole empires by pouring into them masses of troops, but commerce was combined with territorial acquisition, and a continuity of European interest secured by the presence of merchants and settlers.
In 803 and 810 he made a treaty with Charlemagne, by which the limits of the two empires were amicably fixed.
But he had little success, and soon concluded a treaty by which both empires promised toleration to the worshippers of the two rival religions, Christianity and Zoroastrianism.
To the petty states this meant only a change of masters; they now became part of one of the largest empires of antiquity.