The Hagia Sophia is one of the most beautiful and well-known buildings in the world — and Islamists have detested that fact for years. After all, it is a Christian church, and therefore a Christian symbol.
That’s why the Ottoman Turks wasted little time transforming the church into a mosque when they invaded and conquered Constantinople (Istanbul) centuries ago. Christian symbols and works of art were destroyed or covered, and a dominating tower was built from which the Islamic call to prayer could be sung.
The Ottomans did that because they wanted to show Christians that, from then onwards, Islam was in power. Christianity would be subjugated.