In the fourteenth century, the Orthodox Russians realized that the Byzantine Empire had surrendered itself to not just papal authority in Rome but also the Ottoman sultans. They accused the Byzantine Emperors of abdicating their leadership of the Orthodox world by accepting papal and Ottoman overlordship.
The Byzantine Empire of Nicaea had recaptured Constantinople in 1261, but their military power was inadequate for settling the territorial disputes in which they were involved in the Levant and southeastern Europe. Their weakness forced them to solicit military support from the Ottoman sultans. The Ottomans arrived in Europe on the invitation of the Byzantines—in the Byzantine civil war between 1341 and 1347, one of the Byzantine factions used the support of Orhan Ghazi, the Ottoman Sultan, to gain an upper hand in southeastern Europe. Once the Ottomans had got a taste of European politics and warfare, they decided to expand into Europe. In 1354, they captured Gallipoli, and in 1360s, they won Adrianople in the Balkans.
In 1274, Emperor Michael VIII Palaiologos, the founder of the dynasty that would rule the Byzantine Empire until the fall of Constantinople in 1453, pleaded before the Latin Church for military assistance. In 1366, John V Palaiologos visited the Hungarian Kingdom and pleaded for help. The Hungarian King agreed to help on one condition—John V should convert to Catholicism. In October 1369, John V became the first Byzantine Emperor, in seven hundred years, to travel to Rome, where he converted to Catholicism in St Peter's Basilica and acknowledged the pope as the supreme head of the Church. But his acceptance of Catholicism did not hinder John V from accepting the suzerainty of the Ottoman sultan Murad I in 1371. In 1376, Murad I helped John V suppress a civil war.
The Russians were appalled by the Byzantine attempts to merge Orthodox Christianity with Catholicism, and they were suspicious of the Ottoman sultans. In the middle of the fifteenth century, Orthodox Christianity was split into Russian and Greek (Byzantine) branches. The Russians claimed that they were the true representatives of Orthodox Christianity and the inheritors of the Roman Empire. The Russian Emperors started viewing Moscow as the third Rome, and they assumed the title of Caesar (Tsar in Russian).