Answer:
Muslim forces repeatedly attacked the Christian Byzantine Empire.
In 1453, Constantinople fell to the Ottoman Empire. The city was renamed Istanbul
Explanation:
The crusades were arguably started because of the constant attacks of the Muslim forces on the Byzantine Empire. The Byzantine Empire is the medieval version of the Eastern Roman Empire, and as explained in the question, it was a christian state.
In 1453, after a long siege, the Seljuk Turks conquered Constantinople, which had been the Eastern Roman Empire for over 1,000 years. After the conquest, the Turks changed the name of the city to Istanbul.
