In the Early Middle Ages, three different civilizations shared the Mediterranean Sea: the falling Eastern Roman Empire, Byzantium; the rising Islamic regions in Iberia, Northern Africa, Palestine, Arabia; and the Frankish kingdom of the Carolingean.
The fall of the Byzantine Empire
To medieval thought, there was no “Byzantine Empire”: the territory and its citzens considered it as the Roman Empire, being Constantinople the bastion of the Eternal Rome of Constantine. It’s interesting to observe that, even as the official language was Latin, the current language of its population was mostly greek. The citzens of Byzantium considered themselves as Romanoi.
The splendor happened between the 5th and 6th centuries, times when the Byzantine Empire experienced conditions of wealth and expansion to all Eastern Mediterranean, Italy and South Iberian Peninsula.
After the 7th century, the expansion was over and the fall of the empire begun. Damascus and Jerusalem were lost to the Persian Empire, in 613-614. After that, the expansion of Islam, that conquered Syria and Egypt. Slavs and Bulgarians gained part of their territories, Normands conquered Sicily and Calabria.
From the 9th century until the 10th, the Roman Emperors took back their expansion, even conquering Crete, Chipre and momentarily Syria, Palestine and Bulgaria. But after the 11th century, again, the fall increases. The internal structures of power, such as taxing and military, weakened. The external pressures of different powers, such as the Turks sultanates and Bulgarian Empire, made Byzantium’s territories shrink. The Eastern Roman Empire at the 11th century was only a shadow of its former self.
I personally think that the weakening of the Eastern Roman Empire was the main cause to the phenomenon of the Crusades (the first being in 1095-1099). People mostly think the causes to the Crusades at the Western Europe, in France or England. As their internal economic, social, political and cultural of course matters in the historical events of the Crusades, I think that this large military expeditions wouldn’t happen if the Eastern Roman Empire wasn’t falling, and the Christianity (great mark of identity through all Middle Ages) wasn’t threatened.
[And by saying that I don’t mean to justify the violence made by christians against muslims. I just intent to insert the movement in the geo-political context of the Early Middle Ages]
(Main Source: BASCHET, Jérôme. A Civilização Feudal)