The term Third Rome describes the idea that some European city, state, or country is the successor to the legacy of ancient Rome (the "first Rome") and, according to different perspectives, either via connection to the Byzantine Empire (also known as the "Eastern Roman Empire") as being the "second Rome"; or via connection to the Western Roman Empire via connection to its claimed successors such as the Papal States or the Holy Roman Empire as being the "second Rome".
Russian claims 
Within decades after the capture of Constantinople by Mehmed II of the Ottoman Empire on 29 May 1453, some Eastern Orthodox people were nominating Moscow as the "Third Rome", or the "New Rome". Stirrings of this sentiment began during the reign of Ivan III of Russia who had married Sophia Paleologue. Sophia was a niece of Constantine XI, the last Byzantine emperor. By rules laws of inheritance followed by most European monarchies of the time Ivan could claim that he and his offspring were heirs of the fallen Empire, but the Roman traditions of the empire had never recognized automatic inheritance of the Imperial office. A stronger claim was based on religion. The Orthodox faith was central to Byzantine notions of their identity and what distinguished them from 'barbarians." Vladimir the Great had converted Kievan Rus' to Orthodoxy in 989, in return for which he became the first barbarian to ever get an Imperial princess as a wife.
The story of "Third Rome" ("the second Constantinople") started in 14th century Bulgaria, under the reign of Tsar Ivan Alexander. He aimed to raise the prestige of his land and capital, introducing the name Tsarevgrad Tǎrnov for it (in comparison to the Slavic name of Constantinople - Tsarigrad), which was later supported by the words of Patriarch Callistus I of Constantinople that "Tǎrnovo is the capital of the Bulgarians and second both in words and deeds after Constantinople". After the fall of Tǎrnovo to the Ottoman Turks in 1393, a number of Bulgarian clergymen sought shelter in the Russian lands and transferred the idea of the Third Rome there, which eventually resurfaced in Tver, during the reign of Boris of Tver, when the monk Foma (Thomas) of Tver had written The Eulogy of the Pious Grand Prince Boris Alexandrovich in 1453. The idea crystallized with a panegyric letter composed by the Russian monk Philoteus (Filofey) of Pskov in 1510 to their son Grand Duke Vasili III, which proclaimed, "Two Romes have fallen. The third stands. And there will be no fourth. No one shall replace your Christian Tsardom!". Contrary to the common misconception, Filofey  explicitly identifies Third Rome with Muscovy (the country) rather than with Moscow (the city). In addition, Moscow is placed on seven hills, as is Rome and Constantinople. The terms Muscovy (The City) and the boundaries of the Russian Empire were considered both in Russia and abroad to be synonymous at the time.
Ottoman claims 
After the fall of Constantinople, Mehmed II declared himself Kayser-i Rum, literally "Caesar of Rome". The claim was not recognized by the Patriarch of Constantinople, or Christian Europe. Mehmed's claim rested with the concept that Constantinople was the seat of the Roman Empire, after the transfer of its capital to Constantinople in 330 AD and the fall of the Western Roman Empire. Mehmed also had a blood lineage to the Byzantine Imperial family; his predecessor, Sultan Orhan I had married a Byzantine princess, and Mehmed may have claimed descent from John Tzelepes Komnenos. The Ottoman Empire also captured Otranto during that period, and Mehmed II was planning on taking Rome itself when the Italian campaign was cut short by his sudden death. The title fell into disuse after his death, but the imperial bodies created by Mehmed II lived on for centuries to come. The Turkish historian İlber Ortaylı is a proponent of this claim, citing the multicultural make-up of the state and Sultan Mehmed's acceptance of certain Byzantine court customs. Professor Ortaylı finds Russia's claim to the title to be only nominal, and that Sultan Mehmed based his court policies and conquests on creating a third, Islamic Rome (the first Rome being polytheistic, the second one Christian).
After Suleiman the Magnificent defeated the Holy Roman Emperor Charles V he had a treaty signed which addressed the latter with the plain title "King of Spain" rather than Roman Emperor, leading Suleiman to consider himself the true successor to Caesar.
Evidence of this identification can be seen in many cases, such as when Ibrahim Pasha, Grand Vizier to Suleiman, erected Roman-style statues in Constantinople despite it being against Islamic custom, stating that it was natural since the Ottoman Empire was the successor of Rome.
Hapsburg and German claims 
The Austrian Empire sought to lay claim as the heir of the Holy Roman Empire (HRE). Austria's Hapsburgs attempted to unite Germany under their rule. Later the German Empire in 1871 also claimed to be the Third Rome, also through lineage of the HRE. The title for German emperors was Kaiser, the German word for "Caesar".
The Germanic-led Carolingian Empire has been claimed to have deliberately sought to revive the Roman Empire in the West. The Carolingian Empire transformed into the HRE, the HRE was a predominantly German state that claimed to be a continuation of the Western Roman Empire. In 800, the title of Holy Roman Emperor was granted to Charlemagne by Pope Leo III. The HRE for a long period of time was dominated by the Hapsburg dynasty.
After the HRE was dismantled in 1806, Austria's Hapsburgs attempted to unite Germany under their rule.
Claims by the German Empire of 1871 to 1918 of being a "Third Rome" have been criticized because of the fact that the German Empire was led by a Protestant ruler and no concordat had been achieved with the Catholic Church (that was a major basis of the continuation of Roman culture in the West).
Italian claims 
Giuseppe Mazzini, Italian nationalist and patriot promoted the notion of the "Third Rome". He said, "After the Rome of the emperors, after the Rome of the Popes, there will come the Rome of the people", addressing Italian unification and the establishment of Rome as the capital. After the unification of Italy into the Kingdom of Italy, the state was referred to as the Third Rome by Italian figures.
After the unification, Mazzini spoke of the need of Italy as a Third Rome to have imperial aspirations. Mazzini said that Italy should "invade and colonize Tunisian lands" as it was the "key to the Central Mediterranean", and he viewed Italy as having the right to dominate the Mediterranean Sea as ancient Rome had done.
In his speeches, Italian dictator Benito Mussolini referred to his Fascist Italy as a "Third Rome." Terza Roma (Third Rome; the Fascist Rome after the Imperial and the Papal ones) was also a name for Mussolini's plan to expand Rome towards Ostia and the sea. The EUR neighbourhood was the first step in that direction.
See also 
- Legacy of the Roman Empire
- Legacy of Byzantium
- New Rome
- Byzantium after Byzantium (monograph)
- Papal States relationship with the Holy Roman Empire
- Third Reich (not to be confused with)
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