In 595 AD, Byzantine Emperor Maurice sent an elephant and a gold couch to Bayan I, Khagan of the Northeast Caucasian nomadic tribe called Avars, to persuade him to ally with the Byzantine Empire. The gift had an unintended consequence. Bayan I realized that if his nomads squeezed the Byzantines, they would readily pay more. In exchange for his support, he managed to extract a massive tribute from Maurice: 100,000 gold coins, close to 1,000 lbs of gold, per year.
Maurice is believed to be the author of the military treatise Strategikon, a handbook of the Byzantine art of war that has influenced the military tradition of Europe and the Middle East during the Middle Ages, and is valued by contemporary military historians. The focus of the treatise is on fighting the nomads: the Franks, the Lombards, the Avars, the Turks, and the Slavs. The treatise reveals that in the sixth century AD, the Byzantines were deploying nomadic art of war to defeat their nomadic enemies. They had equipped their cavalry with metal stirrups, which were being extensively used by the nomads of the steppes, and they were hiring nomadic horse archers to serve in the Byzantine army.
The Byzantine emperors in the sixth and seventh centuries found it difficult to deal with the nomads because they were at war with the Zoroastrian Empire of Persia, then led by the dynasty of Sassanid Shahs. The war between the Byzantines and the Sassanids over Syria, Armenia, and a few other regions of the Middle East intensified in 568 AD and dragged on for almost sixty years, till 628 AD. Both empires were exhausted as a result of this long war—their army was weakened, their culture went into decline, and their economy was destroyed.
In 590 AD, there was a coup in the Sassanid Empire. The legitimate Sassanid Shah Khosrow II was overthrown by a faction led by Persian General Bahram Chobin. Khosrow II, his wives, and his close political associates fled to Syria, then under Byzantine control. He wrote a letter to Byzantine Emperor Maurice pleading for assistance in regaining the imperial throne of Persia. Maurice agreed to help Khosrow II in return for control over the cities of Amida, Carrhae, Dara and Martyropolis, and the end of Persian interference in the affairs of Iberia and Armenia. Khosrow II also agreed to cede control of Lazistan to the Byzantines.
In 591 AD, the Persian troops loyal to Khosrow II and the troops of the Byzantine Empire jointly marched into Persia. The Byzantine-Persian force defeated the troops loyal to the usurper Bahram Chobin in the Battle of the Blarathon. Khosrow II was swiftly reinstated on the throne of Persia. With 4000 soldiers, Bahram Chobin fled to the steppes where he was given protection by the Khagan of the Turks. But Khosrow could not feel safe till Bahram Chobin was alive. He got Bahram Chobin murdered by bribing the members of the Turkic ruling family.
With the Sassanid Emperor owing his throne to the Byzantine emperor, the relationship between the two empires improved. Maurice was now in a position to focus his attention on destroying the Avars. He passed a decree that his army should winter beyond the Danube, on the steppes of eastern Russia. But this decree was not popular with his soldiers. Maurice did not comprehend the level of discontent that was festering in his army. Instead of paying attention to the grievances of his soldiers, he ordered them to mount an offensive against the Avars. His soldiers mutinied—they proclaimed General Phocas as their new leader. They demanded that Maurice should abdicate. Maurice refused. A civil war broke out in Constantinople. The imperial palace was attacked.
Along with his family, Maurice fled to Nicomedia by a warship. They were captured by the troops loyal to Phocas. On 27 November 602 AD, they were brought to the harbor of Eutropius. Maurice and his wife, Empress Constantina, were forced to watch their five sons being slaughtered. After his five sons were killed, Maurice was beheaded by the soldiers. Empress Constantina and her three daughters were banished to a monastery. With the help of loyalists, they escaped from the monastery but were recaptured and delivered to Phocas who had them executed on the charge of treason.
In 602 AD, Khosrow II attacked the Byzantine Empire. Ostensibly his aim was to avenge the killing of Maurice and his family, but his real aim was to capture as much of Byzantine territory as he could. He regained the territory that he had given to Maurice in return for his help in overthrowing the usurper Bahram Chobin, and then returned to Persia, after handing over the command of his troops to his generals. The Sassanid troops pillaged Syria and Asia Minor, and in 608 AD, they advanced into Chalcedon. In 610 Ad, Phocas was killed by Heraclius who became the new Byzantine Emperor. He made diplomatic overtures to Khosrow II. But Khosrow II refused to negotiate. He regarded Heraclius as a usurper, and wanted to see Maurice’s surviving son Theodosius made the Byzantine Emperor.
The war between the Byzantines and the Sassanids went on for sixteen years. In this period, Khosrow II came close to capturing the Byzantine Empire in Asia. The climax of the war came in 626 AD, when Khosrow II got into an alliance with the Avars and Slavs. The Sassanids attacked Constantinople from the eastern side of the Bosphorus, while the Avars and Slavs attacked from the western side.
The city was saved due to the inspiring leadership of Heraclius. While Constantinople was under siege, Heraclius entered into a pact with the nomadic group of Gök Turks. The Byzantines and the Gök Turks waged a war against the Sassanids and the nomads (the Avars and Slavs) and forced them to retreat from Constantinople. Having saved the capital of his empire, Heraclius went on the offensive. He attacked the Sassanid heartland of Mesopotamia. During a year-long rampage, he devastated the richest parts of Persian territory. By 628 AD, the Byzantine Empire had attained a decisive victory over the Sassanid Empire. Khosrow II was overthrown and murdered by his son Kavadh II who proclaimed himself the new Emperor. He negotiated a truce with Heraclius and brought the war between their empires to an end. Kavadh II executed his brothers and half-brothers to remove all threats to his rule but he died within months of capturing power.
The Sassanid Empire was plunged in chaos and civil war, and became vulnerable to attacks from a new force that had arisen in the region: Islam. Being focused on fighting each other and the nomads from the steppes and Europe, the Byzantines and the Persians had failed to notice the rise of Islamic movements in the Middle East. The Sassanid Empire was under weak rulers, their army was in disarray—they were in no position to mount an effective campaign to control the Islamic movements. In the 639 AD, the Sassanid Empire came crashing down, and Persia was captured by Islam.