One should never forget about the Persians ~ The Eternal Peace between the Roman Empire and Persia is broken after 8 years

When Justinian secured the so-called "Eternal Peace" with the Persians in AD 532 after the Battle of Daras, it is likely that he realized that the peace on his eastern frontier would not actually be perpetual. But he probably thought it would last longer that seven or eight years. In any event, the emperor made the most the respite, gathering his substantial forces from the east which had previously been on station to face down the Persian menace, and readying them for a thrust to the West.

His first target was the Vandal Kingdom which had ruled Roman Africa for nearly 100 years. Squatting upon one of the richest provinces of the Empire, the Vandalic realm had been a thorn in the Romans' side. Their acts of piracy were legendary, extending even to the sack of the city of Rome itself in AD 455. The stars aligned for Justinian in AD 533. The Vandal throne was occupied by a usurper named Gelimer who was reputedly unfriendly to the Romans. Justinian furthered this instability by instigating rebellions throughout the Vandal realm, while at the same time, mustering an army under the command of his bold Master of Soldiers, Belisarius, to strike at the seat of the Vandal government in Carthage. Landing safely in Africa (a trick that previous Roman commanders had failed to accomplish) Belisarius was victorious at the Battles of Ad Decimum and Tricamarum and, having captured Gelimer, he returned to Constantinople in triumph.

Within a year of this signal victory, however, Justinian was already planning an even greater campaign. It so happened that the situation in Ostrogothic Italy was also unstable. King Athalaric, grandson of King Theodoric the Great, had perished before his time in AD 534. The rule over the Kingdom of the Goths and Italians had then passed to his mother, Queen Amalasuntha, but as the Gothic nobles could not endure a woman ruling over them, Amalasuntha accepted marriage to her cousin, a weasel of a man named Theodatus, who was then crowned king. Within several weeks of this arrangement being formalized, Amalasuntha was dead, strangled in her bath. With such a weak character on the Ostrogothic throne, Justinian decided to make his move. He assembled two more armies to menace the Goths. The first, under Mundus, was to invade Dalmatia and threaten Gothic Italy from the north. The second, under Belisarius, would land in Sicily and march up the Italian boot from the south. 

Though things did not go as smoothly as during the Africa campaign, Belisarius was able to resist the full power of the Goths at the great Siege of Rome and then chase their retreating host back to Ravenna. While Belisarius laboriously captured Gothic held fortified cities in central Italy, however, the new Gothic king, Vittiges, hit upon a strategy to relieve his beleaguered kingdom. He would send agents east to convince the Persians to end the Eternal Peace. 

For their part, the Persians had a young king who had taken the throne shortly after the Eternal Peace was originally promulgated. This man, King Chosroes I (or King Khosrow I), had now come of age, secured his kingdom, and was itching for a fight with the hated Romans. As Procopius relates, the Gothic ambassadors found willing ears for their suggestions at the Persian court:

When Chosroes heard [their arguments], it seemed to him that Vittigis advised well, and he was still more eager to break off the treaty. For, moved as he was by envy toward the Emperor Justinian, he neglected completely to consider that the words were spoken to him by men who were bitter enemies of Justinian. But because he wished the thing he willingly consented to be persuaded. [Procopius, History of the Wars, Book II, Chapter II]

Encouraged by the Gothic ambassadors, Chosroes unilaterally ended the Eternal Peace in AD 540 and invaded the Eastern provinces of the Roman Empire.

With the bulk of his forces fighting in Italy and putting down still simmering rebellions in Africa, Justinian was caught with his pants down. Chosroes and his armies burst through the frontier and began demanding ransom from the Roman cities of the east, sacking any that resisted. Meeting hardly any resistance, he besieged the city of Antioch—the fourth largest city of the Empire after Rome, Constantinople, and Alexandria—and successfully captured and looted it in June of AD 540, returning to Persia with a long train of Roman captives.

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So the moral of the story is, "Don't forget about the Persians." And perhaps it's not completely fair to accuse Justinian of forgetting about them. If one reads Procopius, it seems clear that as soon as Justinian and his court were made aware of the dangerous Gothic overture to the Persians, he attempted to bring the war in Italy to an immediate conclusion, offering the Goths a remnant of their kingdom in northern Italy in exchange for peace. 

Belisarius thought this offer to be extravagant given the dire situation of the Goths at the time. How he settled things in Italy, the resulting delay, and the unfolding catastrophe in the East forms the climax of my most recent novel, Belisarius, Book III: Rome the Eternal.