Why did Moscow call itself the Third Rome?

Because after Constantinople fell in 1453, Moscow's rulers and churchmen presented their state as the successor of the Byzantine emperors. The vocabulary of a "Third Rome", with Rome in Italy as the First and Constantinople as the Second, has been used to convey assertions of legitimate succession. In Russia the idea took the form of a narrative, encouraged by successive rulers, that Muscovy was the rightful successor of Byzantium.

The link with Byzantium began with a marriage deal. According to the traditional view, Kievan Rus' was definitively Christianized around 988, when Vladimir the Great was baptized. The conversion likely had its immediate roots in dynastic relationships between Rus' and the Byzantine Empire: the emperor Basil II asked Kyiv for help against a rebel, and Vladimir agreed in exchange for marrying the emperor's sister Anne. When Basil hesitated, Vladimir plundered Chersonesus and signalled that Constantinople could face the same fate, which forced the emperor to concede.

Then came the inheritance. In 1472 Ivan III of Moscow married Sophia Palaiologina, a niece of the last Byzantine emperor, adopted Byzantine court ritual and began to style himself Tsar, meaning "Caesar". In 1492 Zosimus, Metropolitan of Moscow, called Ivan "the new Tsar Constantine of the new city of Constantine, Moscow". In a 1510 letter to Grand Duke Vasili III, the monk Philotheus of Pskov wrote that two Romes had fallen, the third stood and there would be no fourth.

In 1547 Ivan IV took the title Tsar of All Rus, and in 1589 the Patriarchate of Constantinople granted autocephaly to the Metropolitanate of Moscow, which became the Patriarchate of Moscow. Supporters of the idea also pointed out that Moscow, like Rome and Constantinople, had seven hills.