[However Heraclius fought back]. 'Heraclius formed an alliance with the Turks, who took advantage of dwindling strength of the Persians to ravage their territories in the Caucasus. Late in 627, Heraclius launched a winter offensive into Mesopotamia, where, despite the desertion of the Turkish contingent that had accompanied him, he defeated the Persians at the Battle of Nineveh. Continuing south along the Tigris, he sacked Khosrau's great palace at Dastagird and was only prevented from attacking Ctesiphon by the destruction of the bridges on the Nahrawan Canal. Discredited by this series of disasters, Khosrau was overthrown and killed in a coup led by his son Kavadh II, who at once sued for peace, agreeing to withdraw from all occupied territories. Heraclius restored the True Cross to Jerusalem with a majestic ceremony in 629.'

It was these catastrophic wars that had the following consequences. 'The devastating impact of this last war, added to the cumulative effects of a century of almost continuous conflict, left both empires crippled. When Kavadh II died only months after coming to the throne, Persia was plunged into several years of dynastic turmoil and civil war. The Sassanids were further weakened by economic decline, heavy taxation from Khosrau II's campaigns, religious unrest, and the increasing power of the provincial landholders. The Roman Empire was also severely affected, with its financial reserves exhausted by the war, and the Balkans now largely in the hands of the Slavs Additionally, Anatolia was devastated by repeated Persian invasions; the empire's hold on its recently regained territories in the Caucasus, Syria, Mesopotamia, Palestine and Egypt was loosened by many years of Persian occupation.'

To conclude: 'Although warfare between the Romans and the Iranians lasted for seven centuries, the frontier remained largely stable. A game of tug of war ensued: towns, fortifications, and provinces were continuously sacked, captured, destroyed, and changing sides frequently. Neither side had the logistical strength or manpower to maintain such lengthy campaigns so far from their borders, and thus neither could advance too far without risking stretching their frontiers too thin. Both sides did make conquests beyond the border, but the balance was almost always restored in time. The line of stalemate shifted in the second century AD: it had run along the northern Euphrates; the new line ran east, or later northeast, across Mesopotamia to the northern Tigris. There were also several substantial shifts further north, in Armenia and the Caucasus.
The resources expended during the Roman-Persian Wars ultimately proved catastrophic for both empires. The prolonged and escalating warfare of the sixth and seventh centuries left them exhausted and vulnerable in the face of the sudden emergence and expansion of the Caliphate, whose forces invaded both empires only a few years after the end of the last Roman-Persian war. Benefiting from their weakened condition, the Arab Muslim armies swiftly conquered the entire Sassanid Empire, and deprived the Eastern Roman Empire of its territories in the Middle East, the Caucasus, Egypt, and the rest of North Africa. (Over the following centuries, most of the Byzantine empire came under Muslim rule).'