The Ottoman Empire, under Sultan Mehmed II, conquered Constantinople in 1453, captured the Hagia Sophia and converted it into a mosque, and ended the Byzantine Empire. Its legacy of conquest and religious oppression was aimed particularly at Christianity, though it also gave refuge to Jews fleeing persecution in Europe. After taking Mecca and Medina in 1517, the Ottomans claimed the Islamic caliphate, so their regime was recognized as a kind of Islamic capital of the world. For four centuries, from Constantinople (renamed Istanbul), the empire projected its influence across the civilized world, exploiting its position on trade routes linking the Middle East, Asia, Africa, and Europe. After a slow decline, it fell with the Central Powers in World War I in 1918, and the Allies partitioned its territories. In 1923 Atatürk founded the secular Republic of Turkey, turned the Hagia Sophia into a museum, and helped abolish the caliphate in 1924. Erdogan now seeks to revive the Ottoman legacy. He calls Turkey the successor to the empire that spanned three continents for over 600 years, and he promotes "New Ottomanism." He reconverted the Hagia Sophia into a mosque, called it the harbinger of liberating Jerusalem's Masjid al-Aqsa, and welcomed the Muslim Brotherhood, which wants to restore a structure resembling the Ottoman Sultanate. His new defense pact with Saudi Arabia and Pakistan advances an ambition to make Ankara a powerhouse across the old Ottoman territory. If unchecked, such Islamic ambitions foreshadow great trouble for Christians until Christ returns.