To understand why the Battle of Manzikert is a turning-point, one must see first what caused it, and second what it caused.
The Seljuq Turks were a migrating ethnic group from central Asia. Turks had lived in central Asia for centuries. After most of them embraced Islam, they set out for lands to conquer. The Seljuqs - also spelled ‘Seljuks’ - were but one migrating group of Turks. The Mongols were another. The Seljuqs and the Mongols were cousins.
As the Seljuq Muslims moved southwest from their Asian homeland, they eventually encountered the territory of the Byzantine Empire. It is helpful to remember that ‘Asia Minor’ and ‘Anatolia’ and ‘Turkey’ all refer to the same piece of land.
Over a period of years, the Islamic Seljuqs attacked the Byzantine Empire in a number of battles. They also attacked other sovereign territories as they made their way across large portions of southeast Asia.
The Seljuk leader, Alp-Arslan, led his Muslim soldiers not only against the Byzantine Empire, but also against Egypt, Armenia, and Georgia. Emperor Romanos Diogenes led the Byzantines in their attempt to defend themselves. Historian Avner Falk writes:
In 1071, the Seljuk Turks, led by Arp-Arslan (1029-1072), had fought a battle with the Byzantines at Manzikert in eastern Anatolia (now Malazgirt in eastern Turkey), defeating the “Eastern Roman Emperor” Romanos Diogenes, whom they captured, blinded, and exiled to an island in the Sea of Marmara, where he soon died. This battle was an important milestone in the Turkish settlement of Asia Minor. The warlike Seljuks went on to capture Egypt and Syria, including Palestine.
A more detailed telling of the events informs us that Alp-Arslan, whom Falk’s text perhaps misspells, captured the Emperor Romanos Diogenes, held him captive until massive sums were paid for his release, and then returned him to the Byzantines. During his brief stay in captivity, a coup in the palace meant that when Romanos Diogenes returned home, he was no longer emperor. Those who had taken power in his absence had him cruelly blinded.
While the Battle of Manzikert was significant, it was far from the only significant attack mounted by the Muslims. In a large context, the invasion of Anatolia by the Islamic Seljuqs was a continuation of the pattern begun by Islam’s invasion of Spain in 711 AD, and continued by Islamic invasions of France, Italy, and Mediterranean Islands like Malta, Sicily, Cyprus, Sardinia, and Corsica.
Manzikert was, therefore, not the beginning, but rather the continuation, of a long string of Muslim attacks.
Manzikert was also not the end of, but rather one in a series, of subsequent instance of Islamic aggression.
The Seljuqs continued to assault Byzantine territories in Asia Minor for decades and centuries, chipping away at Byzantine civilization, until the capital city Constantinople was savaged by Muslim invaders who largely destroyed the city in 1453 and thereby ended the Byzantine Empire.
At the same time, other Islamic forces continued their attacks on Italy and various portions of Europe. Avner Falk continues the narrative:
Some historians consider the Battle of Manzikert a major cause or origin of the Crusades. A few years later, the Seljuks created their “Sultanate of Rum”, the sultanate that ruled Anatolia in direct lineage from 1077 to 1307, with capitals at Iznik and Konya, and, at times, at Kayseri and Sivas. At its height, the sultanate of Rum stretched across central Turkey from the Mediterranean coast to the Black Sea. In the east, the sultanate absorbed other Turkish states and reached to Lake Van. Its westernmost limit was near Denizli and the gates of the Aegean basin.
After decades of enduring Islamic savagery, Europe had been attacked on many fronts: Spain, France, and Italy. It was clear that an invasion through Greece and the Balkans would be next. If the Seljuq Muslims consolidated their hold on all or most of Asia Minor, then that territory would become their launching pad for Islamic invasions deep into the heart of Europe.
Manzikert was one of many causes of the Crusades; it was not the only cause of the Crusades. The Crusades were a response to long pattern of unprovoked attacks.
Finally, to borrow an athletic metaphor, Europe decided that the best defense would be a good offense. Rather than brace for more attacks, it would be best to go to the source of the attacks, to stop the invaders before they reached Europe. The so-called ‘Crusades’ (they were not given that name until centuries after they ended) were an attempt to go into the Islamic world and there stop the armies of Muslim conquest headed for Europe.