In the seventh century, Persia aggressively expanded westward into the territory of the Byzantine Empire while the latter was in turmoil over the succession by Emperor Phokas. Among other provinces, the Persians occupied Syria and held it for about fifteen years before the Byzantine Empire drove them off.
Persia was at peace with the Roman Empire when the Slavs and Avars invaded the empire in the east in the fifth century. When the two empires were at peace, Persian traders periodically visited Thessalonica. However, no traders were present during the siege of the city. George reflected that, if they had been, their worship of a fire-god may have rendered their fires proof against an Avar spell which extinguished all flames in the city's Christian homes.
For more than two thousand years The Persian Empire and the Roman Empire had existed side by side - rivals which occasionally went to war and conquered some territory from each other, but were neither either able nor really interested in totally defeating the other empire. Wars were almost invariably confined to peripheral areas and did not touch the heartlands of either empire. The introduction of gunpowder and artillery changed the means by which such wars were conducted but not the basic pattern.
On its other flank, Persia had rather similar relations with the westernmost of the two empires into which India was divided.
In between wars, Persia had long periods of peace with its neighbors, during which there were considerable trade and significant cultural influences.