Warmatrix

The wars between two western empires, Roman and later Byzantine and two eastern ones, Parthian and later Sassanid, was a series of conflicts that lasted a staggering seven centuries. Early on Roman expansion to the east was checked by the Persians, who were likewise kept from moving westward. Because both empires were expansionist. They did not let off even after centuries of wars, severe losses and almost zero gain.
Since its beginning, the Roman republic had grown steadily, a process that was accelerated when it set up a professional army. The need to subdue one aggressive neighbor after another, but also the prospect of wealth and prestige, kept the expansion going. Eventually the Roman legions moved into Asia Minor and started breaking down the Seleucid empire from the west. On the other side the Parthians asserted themselves and conquered Seleucid territory from the east, effectively re-establishing a Persian empire that had been broken by Alexander the Great. Soon the two superpowers met at the river Euphrates in Mesopotamia.
The Persian empire was shielded on three sides by seas and mountains, leaving the west as its only area of expansion. Rome on the other hand wanted control of the rich trade routes that ran through the nucleus of the Old World. What followed was a series of raids, skirmishes, battles and sieges, from time to time becalmed by peace treaties and indemnities, but always to flare up again. Most battles were inside and around the plains of Mesopotamia in the south and mountainous Armenia in the north. The border shifted back and forth over the centuries, but never far; in fact it remained remarkably stable.
The Parthian military, rallied quickly in times of crisis, was well suited for defense, but less for offense. The Romans, with a capable standing army, initially had the upper hand. But when they penetrated the Iranian plateau, the Persian heartland, they could never hold on to their conquests. They were vulnerable to Persian horse archers and heavy cataphract cavalry. Later the Romans adopted cataphracts too, while the Persians started to use Roman siege techniques. The Sassanid empire, which was more centralized than the Parthian, gradually reversed the situation and got the upper hand over the Romans and Byzantines. But they too could not gain a decisive advantage. The chronology of the wars is telling.
The first war was in the 1st century BCE. In 69 BCE Rome attacked the kingdom of Armenia, while Parthia remained cautiously neutral. That changed when the Roman consul Crassus invaded Mesopotamia. He suffered a shattering defeat in the Battle of Carrhae and for a while Rome turned its attention elsewhere. The Parthians intervened several times in the civil wars of the Roman republic, sometimes supporting one Roman over another. They raided Syria when opportunity arose and invaded when Rome was weak, but could never conquer that land. In Armenia they were more successful, often ruling that country indirectly as a vassal state. In the 1st century CE there were full scale wars and feeble peace agreements. In the 2nd century CE there was a fresh series of conflicts, Rome having the upper hand. In 115 BCE the Roman emperor Trajan even managed to capture the Parthian capital Ctesiphon, but faced several revolts and his successor Hadrian returned to a status quo. In the second half of the century fighting once more erupted and again Ctesiphon was sacked, but the Parthians struck back without intent by spreading the Antonine plague to Roman territory. In the early 3rd century CE the Parthian dynasty fell but was quickly replaced by the Sassanid. Wars continued as before. For a time Rome, soon replaced by Byzantium, was the stronger power, defeating the Persians at the Battle of Satala in 298 CE and gaining Armenia, parts of Mesopotamia and the royal Sassanid harem. But later they had to secede all their gains in exchange for a safe retreat of their emperor Jovian, after his predecessor Julian had been killed in battle. Only in the 5th century CE the front was relatively quiet, because both empires were beset by problems elsewhere. In the 6th century new wars broke out. An 'Eternal Peace' agreement was signed in 532 CE, yet lasted onyly eight years. In the 7th century the Sassanids finally gained the upper hand and the Byzantine empire nearly broke down, though still managed to strike devastating raids into Persian territory.
The almost continuous warfare left Persia overtaxed, economically weak, disturbed by religious unrest and dynastic struggles. The Byzantine empire was in an even worse state, its treasury also empty, having lost almost all its territory in the Balkans and Anatolia pillaged into poverty. When the Muslim conquest rose up from the south, the Sassanid empire crumbled quickly and completely. The Byzantines were pushed back but survived and hung on for another seven centuries.