Why did the Byzantine Empire finally fall?
The Byzantine Empire was centered right at the crossroads of Europe and the Middle East. It was a strong trading city because of this, so it was highly coveted. After the Great Schism in 1054, the Byzantine Empire fell from its major empire status to a minor empire, no longer able to call upon help from the West. On its Eastern front, Islamic Turkish invaders were pressing on the Byzantine border. By holding all lands to the East, the Seljuk Turks cut off trading lines between Constantinople (the capital of the Byzantine Empire) and Asian provinces. The Byzantines lost tax revenue, food supply, and trading opportunities.
In 1071, the Byzantine Emperor lost a skirmish against the Turks at the Battle of Manzikert. A large part of his army was killed, and it never regained its former strength. Sensing the Byzantine Empire was losing its ability to rule a centralized region, northern provinces started breaking off into independent Slavic states.
At this point, the Byzantines tried to call for aid from its Western brethren, but their request was ignored. Italian merchant city-states sailed to Constantinople, taking advantage of special trade agreements. This made the western merchants turn their focus on the East. In 1204, Italian merchants went on a crusade that initially started on trying to conquer the Holy Land, but ended up focusing on taking down Constantinople for its profits and access to Eastern trade. While the attempt failed, it certainly highlighted Byzantine weakness.
Ultimately, this few hundred year period of weakness and military challenged culminated in 1453 with a large Turkish army that conquered the city. The Byzantine Empire had finally fallen, and Islam had spread further into Eastern Europe.
The Byzantine Empire fell in 1453. The immediate cause of its fall was pressure by the Ottoman Turks. The Ottomans had been fighting the Byzantines for over 100 years by this time. In 1454, Constantinople finally fell to them and their conquest of the Byzantine Empire was complete.
Ironically enough, the major cause of the decline of the Byzantine Empire (what made it weak enough to fall to the Ottomans) was the Crusades. The Crusades were supposed to be Christian wars against Muslims. But the Western Europeans could not resist getting involved in the Byzantine Empire. In 1204, the Fourth Crusade conquered Constantinople, partly because the Venetians wanted to eliminate the Byzantines as competition in trading matters. Constantinople did not return to Byzantine hands until 1264. This weakened the empire significantly and helped cause its eventual fall.
According to history, it is inevitable that all empires should fall. Usually, empires are too powerful to be taken down by outside forces. For this reason, most empires fall from within. While the Eastern Roman Empire did have to contend with a powerful Turk empire to its east, and re-emergence of Western Europe, it seems that internal decay caused its collapse. One of the great strengths of the Byzantine Empire was the sheer size and organization of its military. The various provinces were required to send a certain amount of troops. This was known as the theme system, and it allowed Byzantine emperors to efficiently mobilize large numbers of troops in very short order.
Two civil wars in the early 14th Century severely diminished the military capabilities of the emperor. The emperors only had themselves to blame for the consequential decline in their military. The first civil war (1321-1328) was fought between the emperor and his grandson. The two sides fought for seven years with the result that they would share power. While this imperial family feud was happening, the Turks were watching. By the end of the war, the Turk capital was less than one hundred miles from Constantinople.
A second civil war occurred in 1341 and lasted for over six years. This was a class war between the throne and the nobility. This time, it was the Serbs that used this civil conflict to their advantage, racking up significant territorial gains.
The end result of these civil wars was that the Byzantine Empire had destroyed a very efficient military system, lost significant population, and seriously disrupted its trade network and economy. While the empire was on the decline for some time before these events, the civil conflicts hastened its demise.