By the final years of Justinian's reign, the heady era of Belisarius's western reconquests was only a memory. In 542, a devastating plague—very likely an earlier strain of Y. pestis, the bacterium behind the medieval Black Death—swept through the East and killed a third or more of the population. It took the economic and military stuffing out of the Empire, forcing it to buy an expensive peace from the Sassanid king Khosro I in 545 and leaving it unable to check the ravages of the nascent Slavs and Bulgars in the Balkans. Constantinople was on the defensive.
The emperor himself hadn't been the same since his wife Theodora had died, probably from ovarian cancer, in 548. Without the counsel and support of the woman who had convinced him not to abandon his throne during the Nika riots in 532, Justinian showed little interest in governing his shrinking, increasingly destitute empire. He instead preoccupied himself with the theological disputes for which Byzantium would become so infamous to later historians. It took barbarians at the gate to bring his focus back, if only temporarily.
In 559, about 4,000 Kotrigurs (probably a Bulgar group) crossed the Danube, marched through Thrace and came within 20 miles of the capital which, but for its massive walled fortifications, was nearly defenseless. Civilians fled across the Bosporus into Asia Minor. Justinian played his oldest, best hand, calling Belisarius in from pasture and charging him with neutralizing the Kotrigur threat.
Belisarius, by this time well into middle age, didn't have much to work with. He rounded up 300 of his former veterans, who probably weren't much younger than he was, and supplemented them with militia mustered from the city and its immediate environs. He ambushed the Kotrigurs on their approach to the city, inflicting 400 casualties on them and sending the rest into retreat.
Justinian, who had always loved Belisarius as long as he didn't get too popular, rewarded him by dismissing him, assuming command of his "army", and holding a triumphal parade for himself. A few years later, Belisarius was imprisoned on trumped-up charges of conspiracy against Justinian, who let him rot for eight months before restoring his honor to him in 563.
The two men, who between them had briefly appeared as though they would restore the Roman Empire to its ancient glory, died a few months apart in 565. Within five years, Italy, with the exception of Rome, Ravenna, and some minor outposts, was lost to the Lombards. It would never be in imperial control again. By 584, all that remained of Belisarius's reconquered territory was North Africa, which would hold out until the Islamic onslaught of the 7th century.
On that cheery note, a more whimsical look at Belisarius's last hurrah.