Under Emperor Justinian, with the help of his general Belisarius, the Eastern Roman Empire nearly reconquered what had been the Roman Empire, in the 7th century. But this was the height of Byzantine power, & the Empire would only grow smaller, thereafter. The Moslems assailed the Byzantines in the Middle East, capturing Egypt, northern Africa, & eventually Palestine & Syria. In the Balkans & Illyria, Germanic, Slavic & Altaic invaders pressed the edges of the Empire. Over the course of centuries the Empire was squeezed, and grew ever smaller.
With so many enemies ringing it, the Empire managed to cling to life by playing rival groups off on each other. They raised foreign policy to an artform, & developed a finely-tuned form of statecraft which was quite ingenious. This was documented by Emperor Constantine VII Porphyrogenitos, in a memoir he called De administrando imperio, or "On the administration of the Empire." But, as foreign policy grew subtle & complex, so too did domestic politics. Factions of the Empire rivaled one another, constantly trying to gain authority at another's expense. The office of Emperor changed hands sporadically, usually through palace revolts led by generals who used their armies to enforce their authority — just as Julius Caesar & so many other classical Roman Emperors had done.
Byzantium's prominence & importance lay in its location, at the crossroads of two continents (Europe & Asia). A trading center from the time of its founding some 5 centuries BCE, it remained a commercial center ever after, & therein lay both the source of its power, & the cause of its vulnerability. Trade within & around the city provided tariff revenues for the Emperors; at the same time, it seemed a gold mine for potential conquerors.
Another occasion during which Byzantium was almost destroyed, was the Fourth Crusade. The leaders of the Fourth Crusade were heavily in debt with the merchants of Venice, when they financed their expedition to the Holy Land. As partial payment for that debt, the Venetians demanded that the Crusaders attack Constantinople, their main competitors in commerce. After capturing the city, seizing the Emperor, & replacing him with one of their own (Baldwin Count of Flanders), the Crusaders became embroiled in Byzantine politics. Some Byzantine factions allied with the westerners, & cooperated with their new regime. The descendants of the Comnenus family, which had held the Imperial Crown for almost two centuries, fled to a Greek city in northern Anatolia, called Trebizond, & set up a rival Empire. Another faction in the Greek mainland set up its own Empire.
Eventually the Crusaders were driven out & the Fourth Crusade came to a rather inglorious end, without ever coming near the Holy Land that they'd ostensibly planned to save. The Imperial Crown returned to Byzantine hands — although the new state at Trebizond lived on (it maintained friendly relations with the rest of the Empire & could be called a subject state).
The Osmanli, or Ottoman, Turks & their coalition drove the Byzantines almost completely out of Anatolia by the end of the 14th century. In 1402 they had crossed into the Balkans & taken up residence to the north of the Greek mainland, not far from Constantinople, & in fact laid siege to that magnificent ancient city. But at about the same time the Tatars, under their great leader Tamur (or Timur, or Tamurlane, the subject of a pair of plays by Shakespeare's contemporary Christopher Marlowe) swept into Anatolia & attacked the Ottomans from the east. They had to raise their siege of Constantinople, to meet this threat, & Tamur defeated them. It took several decades for the Ottoman regime to rebuild itself & move its capital back to the Balkans, to resume their war on the Byzantines.
On a number of occasions, Christian realms within Byzantium's sphere of influence, such as Hungary & Serbia, came to Constantinople's defense. But the Serbs were defeated at the Battle of Kosovo, & the Hungarians were unable to stop the Ottomans. When he came to power in 1451, the Ottoman sultan Mehmet II immediately began his campaign against Constantinople.
The Trebizond Empire of the Comnenus family, oddly enough, had managed to survive the initial growth of the Ottoman regime; but within a decade after the fall of Constantinople, they were overcome, & became subjects of the Ottoman Sultan. That line of Emperors, too, came to an end.
The city's impending doom, just prior to its fall, had encouraged even some of its oldest enemies — the Italian trading centers of Venice & Genoa — to rally to its defense. (They feared the Ottomans taking the place of the Byzantines as their competitors — they didn't want the Byzantine fleet to fall into the hands of the "savage" Turks.) Neighboring states such as Serbia & Hungary, with whom the Byzantines hadn't always been friendly, had similarly come to its aid. No one, not even most of Byzantium's enemies, it seems, wanted the glorious city of Constantine to fall.
When Constantinople was sacked, the sound reverberated around the world. While the "Roman mandate" had moved to Moscow, the original Roman state — in its heyday the greatest empire the world had ever seen, a state which had endured for some fifteen centuries — came to an end. This did not go unnoticed. Constantinople's fall was lamented far & wide; many reacted with shock & even disbelief. The Pope sent emissaries there, just to find out if it really had fallen.
So profound was the respect for the Byzantine state, that the Ottoman sultans did not dismantle the Empire's government. Rather, they assumed control of it, and maintained the bureaucracy which had taken a thousand years to build up. Of course, this led to some problems, particularly under weak Sultans, but it appears the Ottomans wanted to have pretty much the same kind of state the Byzantines had ruled.
The Ottoman state endured into the 20th century, after having gotten involved in World War I as an ally of Germany & Austria-Hungary. As a result of that War, the Ottoman Empire lost a good deal of territory — including nearly all of its Balkan lands, Syria, & Palestine. Shortly thereafter the office of Sultan was set aside, the capital moved to Ankara in the interior of Anatolia, & it became the modern republic of Turkey.
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