The Arsenal

The elite of the Persian army was the savaran — heavily armoured noble cavalry, man and horse encased in mail and lamellar, wielding the long lance (kontos), bow, sword, and mace. These aristocratic knights delivered the shock charge that was the Sasanian army's decisive arm, and their prestige and skill were unmatched in the East.

The backbone of Heraclius's reformed army was the versatile heavy cavalryman codified in the Strategikon — armoured, and armed with both composite bow and lance, able to skirmish at range and charge home. Heraclius used this flexible cavalry, and the disciplined all-arms tactics of the Byzantine manuals, to outfight the Persians in his counteroffensive.

The triple line of land walls protecting Constantinople — a moat, an outer wall, and a massive inner wall studded with 96 towers — was the most formidable fortification in the world. In 626, with the emperor away, these walls withstood the combined assault of the Avars, Slavs, and Persians and saved the empire from extinction.

The Byzantine fleet's command of the Bosporus and the Golden Horn was decisive in 626: it sank the Slav dugout canoes trying to ferry the Persian army across the strait and kept the two besieging armies — Avars on the European side, Persians on the Asian — from ever uniting. Divided by the water, neither enemy could take the city.

The Sasanians fielded war elephants brought from India, used as mobile fighting platforms and to terrify and trample enemy formations — the 'moving towers' of the Persian army. They served in the great battles and sieges of the war, though against disciplined and well-led troops they could be turned into a dangerous liability.

Both empires fielded masses of light horse-archers, an inheritance of centuries of steppe warfare, who harassed, skirmished, and softened enemy formations before the heavy cavalry charged. The Sasanians in particular deployed large numbers of mounted archers alongside their savaran lancers.

The Persian conquest of the Roman East was a war of sieges, and the Sasanians were expert besiegers, using rams, siege towers, mining, stone-throwing engines, and blockade to take great walled cities like Antioch, Jerusalem, and Alexandria. The Avars brought their own advanced siege engines to the 626 assault on Constantinople.

Heraclius's masterstroke of diplomacy was his alliance with the Göktürks, a powerful Turkic steppe empire, who invaded Persia from the north through the Caucasus in coordination with his own offensive. Their swarms of horse-archers helped pin and distract the Persian forces and opened the road for Heraclius's deep strike.

The armoured cavalry of both empires wore extensive mail and lamellar armour, with the Sasanian savaran and Byzantine cataphracts among the most heavily protected troops in the world, sometimes armouring their horses as well. This armour made the heavy cavalry charge the dominant tactic of the age.

The long Roman–Persian frontier was studded with great fortress cities — Dara, Amida, Edessa, Nisibis — whose possession had been fought over for centuries and whose walls anchored each empire's defence. Their capture and recapture was a central object of the war's campaigns in Mesopotamia and Armenia.