The city built by the Emperor Constantine the Great on the site of the Greek city of Byzantion (Byzantium to the Romans) as capital for the Eastern Roman Empire – the new Rome.
It was capital for the Roman and Byzantine empires, 330-1204, then of the Latin Empire until 1261, of the restored Byzantine Empire, 1261-1453, of the Ottoman Empire, 1453-1922, and was the seat of the last Caliph, 1922-24.
It fell twice, in 1204 and 1453, earth-shaking events in both cases. In 1920, by the Treaty of Sèvres, European Turkey was confined to the city and its immediate hinterland, up to the Chatalja lines.
This was also the practical limit of the Sultan’s authority at that time, a new nationalist government having been established in Anatolia. He even had to share authority in this relic of an Empire with the Allied forces in the city of Constantinople.
The whole fabric of the Sèvres settlement was beginning to collapse even as it was being put into place, and by 1923 the city of ISTANBUL was part of nationalist Turkey, though no longer the capital of a state, merely of a small province bearing its name and straddling the Bosphorus.