The Romans were down, but they were not out. The Western Roman Empire had collapsed, but the wealthier Eastern Roman Empire was still around. The Eastern Romans were to become known to later historians as the Byzantines, but that is not how they or their contemporaries thought of them. To some extent they were still around, because they had more physical barriers between them and their enemies, but they had reorganized and were in the process of retaking the lost Western Empire, as well as making gains in the East. Why didn’t they succeed? What happened to them?
And finally there is a broader question, why was there so much chaos and collapse across the world. Why did Asiatic horsemen, Central American Empires, The Achaemenid Persians all collapse with them.
There are two related answers to the question.
The first answer is that there was a volcanic eruption that was large enough to darken the skies and change climate patterns for an uncertain number of years.
David Keys in Catastrophe: An Investigation into the Origins of the Modern World, believes the prime culprits are two calderas just under the sea surface between Java and Sumatra in Indonesia not far from Krakatoa. Krakatoa is of course the volcano that caused the world to darken in what became known as the year with no summer.
In the Encyclopedia of Global Warming and Climate Change (Vol. 1), there was some sort of major climate event around 536 CE (Christian Era) is not much in doubt. Tree ring analysis in Ireland, Scandinavia, California, and Chile all show very little tree growth during this period.
Joel Gunn at University of North Carolina Greensboro noted folk lore from the British Isles and tree ring and ice core data from England, Sweden, Finland, Siberia, china, and Korea. Drought occurred in Peru, East Africa, India, China, and Korea. Gunn has also unearthed an account from a praetorian prefect in Italy to his subordinate in 536 CE:
Just as there is a certain security in noting seasons recurring at their proper times, so likewise we are filled with great curiosity when such events seem to be altered. What sort of experience is it, I ask you, to look upon the principal star and not to perceive its usual light? To look upon the moon-the decoration of the night- in all its fullness but without its natural splendor? We all perceive a blue colored sun. We wonder that at noon bodies do not have shadows, that the strongest heat has reached the inertia of extreme tepidity, because –not by the momentary failure of an eclipse, but for the space of an entire year-it has failed to be fixed in its course….Thus we had winter without storms, spring without moderate temperature, summer without heat. How can we hope for a temperate climate when the months which could have ripened the fruits froze them instead by its northern blasts? How can the earth provide fertility if it is not warmed by the summer months? How can the grain sprout if the soil as had no rain” From Joel Gunn, The Years without Summer: TRACING A.D. 536 AND ITS AFTERMATH (Brain Damage, Behavior, and Cognition,)
The grain did not sprout, and people dead by the millions.
The effects of this were felt in what was to become known as the New World. Teotihuacan was a mid-sized Empire in Central America. The earliest city construction is around 100 BCE, and the largest monumental structures may date to around 100 CE.
The city reached its peak in 450 CE, when it was the center of a powerful culture whose influence extended through much of the Mesoamerican region. At its peak, the city covered over 30 km² (over 11½ square miles), and probably housed a population of over 150,000 people, possibly as many as 250,000. Various districts in the city housed people from across the Teotihuacan region of influence, which spread south as far as Guatemala. Notably absent from the city are fortifications and military structures.
Scholars had thought that invaders attacked the city in the 7th or 8th century, sacking and burning it. More recent evidence, however, seems to indicate that the burning was limited to the structures and dwellings associated primarily with the elite class. Some think this suggests that the burning was from an internal uprising. They say the invasion theory is flawed because early archaeological work on the city was focused exclusively on the palaces and temples, places used by the elites. Because all of these sites showed burning, archaeologists concluded that the whole city was burned. Instead, it is now known that the destruction was centered on major civic structures along the Avenue of the Dead. Some statues seem to have been destroyed in a methodical way, with their fragments dispersed.
Evidence for population decline beginning around the 6th century lends some support to the internal unrest hypothesis. The decline of Teotihuacan has been correlated to lengthy droughts related to the climate changes of 535-536 CE. This theory of ecological decline is supported by archaeological remains that show a rise in the percentage of juvenile skeletons with evidence of malnutrition during the 6th century. This finding does not conflict with either of the above theories, since both increased warfare and internal unrest can also be effects of a general period of drought and famine. Other nearby centers such as Cholula, Xochicalco, and Cacaxtla competed to fill the power void left by Teotihuacan's decline. They may have aligned themselves against Teotihuacan to reduce its influence and power. The art and architecture at these sites emulates Teotihuacan forms, but also demonstrates an eclectic mix of motifs and iconography from other parts of Mesoamerica, particularly the Maya region. Wikipedia entry on Teotihuacan.
The classical Mayan Empire was to be built on the ruins of the Teotihuacan.
In 536 CE the Slavs, farmers who lived in what is now Poland and the western half of the Ukraine, began a series of invasions across the Eastern Roman's borders. These invasions were to continue until some new people arrived on the scene.
On the Asiatic steppes, the Avars, Asiatic horsemen that had been the dominant player on the Steppes for about 150 years, began to have problems. When severe droughts occurred after the volcanic induced climate cooling, cattle, with their more advanced digestive systems were more able to make use of the scarcer fodder. With their horses beginning to die off their power began to stumble. Avar power was collapsing on the Steppes.
The Avars were eventually forced to choose between losing their power or flight. They choose flight. They arrive at the edge of the (Byzantine) Roman Empire around 557 A.D. They had fought their way across, but had also conscripted some new troops along the way: the Slavs. Having started out as a weakened group, by the time they arrived they were strong enough to start demanding tribute from the Romans. Although the term slave did not come into the English language for some time, the period of equating the slavs as slaves was to begin with the arrival of the empire building Avars.
In other parts of Asia, the Chinese had frost in July of 537 and snow in August (Keys). In Southern China the tax system collapsed as people were dying. Popular revolt broke out. In Japan, following the weather induced famine, a devestating pandemic of what was likely small pox broke out.
So at the end or our second “answer” we have collapses and destabilization, but nothing that would be thought of as calamitous.