The popular conception - a conception that I still held until recently - was that the western half of the Roman empire collapsed in the 5th century, and that the eastern side of the empire - the Byzantine empire - would survive for another 1,000 years. On its face this is true, but in the 7th century the Byzantine empire experienced a collapse even more dramatic and traumatic than the western one; survival can look like a lot of different things, depending on the particular story.
As always this incredible story was long and complex, but in a rough outline, the ambitious Roman drive to expansion remained even after the terrible convulsions of the 5th century, in which Western Rome fell and Eastern Rome struggled to fortify its remaining possessions after cutting its losses.
Eventually the east would rebound and make a bid to reconstruct the empire, as its citizens hadn't stopped believing in their Roman identity. However, the reconstruction would lack the continuity - at least in terms of sheer landmass of the original Roman Empire - reminiscent of the past glories.
The emperor Justinian in particular was a famous emblem of overreach: he was an inexhaustible personality that pursued relentless reforms, civic engineering projects (the Hagia Sophia being one of them), and foreign adventures to reconquer the lost territories of the empire, with the added resistance of the doubts and skepticism of Justinian's inner circle and subordinates. The doubt and skepticism would prove prescient: much like with Diocletian's character in the late third century, much of the vitality of the reformist fervor would be localized to the individual, in contrast to deep cracks and weaknesses in the greater system, which would eventually overtake the gains afforded by the reforms.
In the short run, these spectacular conquests lent to a bright, sparkling extension of a reconstructed and even improved Roman Empire, an extension that hung precariously in the air no doubt. The saga of the Hagia Sophia would prove to be a milder and lower-stakes microcosm of things to come: as spectacular and awe-inspiring as the actual structure was, the project was rushed and the dome walls would not be given enough time for the mortar to properly set, causing them to lean, requiring them to be rebuilt. In addition there were several other architectural flaws that made themselves visible when a series of earthquakes caused cracks to form in the main and eastern domes, with the eastern dome partially collapsing at one point, so that they had to be reconstructed as well.
Similarly, the spectacular, sparkling reign of Justinian would give way like a flashing chunk of magnesium catching fire, after domestic unrest, a series of earthquakes, and early waves of the Black Plague itself would wipe out huge swathes of the population, and an already tenuous imperial extension would be weakened further. And then in this period of vulnerability, the Arab invasions erupted in North Africa - which thundered into the region like the German blitzkriegs, but which by contrast produced a durable and lasting empire - wiping out huge swathes of Byzantium's precariously reclaimed holdings in the east, resulting in a slew of terrible military losses and attendant casualties, and collapsing the empire back down into the rough footprint of Constantinople and the surrounding region.
The accumulated population loss and the catastrophic collapse in imperial holdings - which included Egypt, a crucial strategic source of grain and geopolitical advantage - meant a profound disintegration of wealth and standing, which triggered a sea change in political and religious ideology and self-conception, which I'll try to touch on later.
There were a number of reasons that the Byzantine empire survived this, but the denser kernel residing in the middle of it all was the geographical and material fact of the city of Constantinople itself, which proved to be a durable material shell protecting the greater socio-economic and political continuity of the empire, and which had and would for centuries nurture a steady stream of refugees from the late classical collapses, developments of knowledge and intellectual endeavor, a modernized bureaucracy and its attendant pathways to stable professional and civic life, a modernized legal code that would be adopted throughout Europe, and so on.
In an era in which conventional warfare decisively determined the continuity and fate of empires, it was the superior material fortification of the city itself that provided the physical and then cultural and spiritual continuity of the empire, which retreated into the hard shell as all of its extended geopolitical commitments collapsed all around it.
The city of Constantinople rested on a peninsula, surrounded by the Sea of Marmara, with a protective inlet curving up and to the northwest, known as the Golden Horn, which would protect centuries of military and commercial ships. The sea-facing sides of the peninsula were fortified with walls and gates, and the entrance to the Golden Horn was blocked off by a huge iron chain, which allowed the city to control its maritime traffic, regulate the entrances to its harbors, offer protected points of resupply, and maintain its eastern trade networks.
At the same time, there was only one direction that the city could be entered by land, and this limited entrance afforded resources to be concentrated into the daunting Theodosian Walls, which with their three barriers of increasing height, separated by swatches of assailable no man's land, proved to be insurmountable for centuries, until the Turks had to roll up a giant cannon and blast a hole in them to finally enter the city. At that point the early Ottoman Empire rushed into that broken shell and situated itself within.
All of that pressure and turbulence, two thousand years of expansion and collapse; that was a tough nut to crack. For as long as you can manage to keep a smoldering core of coals, you can always spark up another flame.
