It first started in 568, when the Lombards attacked Italy. The Byzantine Empire was not at all prepared to do anything about it. Large areas of Italy were quickly occupied by Lombard forces, who established duchies in the captured regions. The Romans could not muster any more armies to sail off to Italy and attempt to reclaim the province once more, and at this point, there was no point.

This initial Lombard attack was motivated by the increasing power of the Avars across the Hungarian plain after they had conquered several tribes and expanded their influence into former Lombard land. Just as the Huns drove the Goths west into Roman territory, the Avars drove the Lombards west into Roman territory. The Roman presence was weak and quickly withdrew to defensible cities, leaving much of northern and central Italy for the taking.

Over the following centuries, Rome would slowly lose control of its remaining cities in Italy. Rome broke from Constantinople’s authority some time in the 700s, and Venice and Ravenna by the year 800. Rome did launch a few campaigns to reestablish their holdings in Southern Italy (traditionally Greek “Magna Graecia”), making the region into an economically productive hub for trade, but it too was lost, after the disaster at Manzikert, to the Normans.

So why did this happen to Rome? Why were there no major battles fought over control of Italy, why are we unclear on when exactly major cities left the Roman sphere of control, why were the Romans not standing fast and holding down the fort in their ancestral province?

In the beginning of the answer, I stated that the fall of Italy started with the Lombards. I lied. It actually started with the reconquest of Italy.

Under the emperor Justinian, the Roman Empire undertook a series of military campaigns aimed at retaking several politically and economically crucial territories that had been lost in the fall of the West. Over the course of almost thirty years, the regions of North Africa, Italy, and part of Hispania were reclaimed by Roman authority and made provinces in the Roman Empire. Justinian’s campaigns brought the redefined Eastern Roman polity to its greatest territorial extent.

But they weren’t supposed to last thirty years. They were supposed to last, like, less than a decade.

North Africa was reconquered by the general Belisarius in about a year’s time. Italy was a harder nut to crack due to the terrain and the greater strength of the Ostrogothic authorities that controlled it, but it too fell in about five years. By 540, Belisarius had pushed all the way to the passes of the Alps and was negotiating terms of final surrender with the Goths in Ravenna.

Unfortunately, Roman forces had been overextended in the East. Sassanid Persia took advantage and attacked, sacking the great city of Antioch. The results in Italy were soon challenged by several Gothic revolts, but before Belisarius could cross the Po River into the Gothic heartland and snuff out the embers, Justinian recalled him to the Eastern front, leaving the Goths matches and firewood with which they would start a great conflagration.

The war that followed was one of the bloodiest and longest that the Romans had ever fought. Under the leadership of a new king, Totila, the Goths recaptured city after city, taking Verona, Naples, Florence, and then Rome itself. Belisarius was sent back to reorganize the remaining forces, but there was very little remaining to reorganize. He did reoccupy Rome, but he did not have the strength to hold his gains and returned to Constantinople in 548. By 551, the Goths had pushed all the way to Sicily and were raiding the coast of Greece from their ports on the Mediterranean.

During this period, the entirety of Europe was hit by a devastating epidemic of black plague. The Roman Empire, with its heavily concentrated urban centers, was hit the hardest. Millions died in cities, villages, and towns. Economic activity ground to a halt, settlements were abandoned, infrastructure was neglected, people starved. Populations were severely reduced, and those left had little with which to rebuild their lives.

In 552, Justinian, having made peace with the Persians and with the worst of the plague behind him, ordered a new general with an army to retake Italy. The Goths were defeated in open battle within a couple years, but it once again took until 561 before the final Gothic revolt was quelled. Victory at last.

But it was a horribly empty victory. Looking back on whether Rome was better in 530 or 560 wasn’t even close. The East had been ravaged by a decade of war with Persia, Italy and North Africa were severely depopulated by war and instability, and the whole Empire was reeling from the costs of the lengthy campaigns, both financially and demographically. On a larger and more unavoidable scale, the plague that bore Justinian’s name had devastated the entire Mediterranean, leaving the Roman Empire with weak armies and dwindled resources.

Northern Italy in particular, the area taken over by the Lombards, failed to recover from the dislocating effects of the warfare, in the sense that there is almost no evidence for complex exchange systems, and the admittedly incomplete evidence is suggestive of considerable population decline. No figure can be put on the latter, but what had been a great hub of the late Roman world both declined in demographic terms, and saw its economy move decisively towards only very local exchange.

—Peter Heather, The Restoration of Rome

In this situation, was it any surprise that the Romans were too weak to oppose the migration of an entire tribe into a territory that was economically devastated by decades of war?

They couldn’t have known it at the time, but the Romans were destined to lose Italy from the moment they tried to take it.