Why was Turkey allowed to keep Constantinople after WW1? Greece was on the winning side, weren't they? Wouldn't it have made sense for the European great powers to support a Greek demand on Constantinople? After all, the Greeks are European, and Christian, and the European culture and civilisation traces its roots back to ancient Greece.
If you consider the reason for WW1 (an assassination of an individual that culminated in declarations of war based on alliances), these were not conflicts to gain territory. It would be unfounded to claim a city with Turkish inhabitans from Turkey as reparations.
Part of Turkey is also European. Some Turks are Christians too. And I don’t see how the roots of European culture are traced back to Ancient Greece. European peoples had their own cultures. Ancient Greece had an influence in spreading civilization. But would these influences be reasons to support a hypothetical illegal claim on a foreign city by Greece? That makes no sense.
Constantinople was founded by the Roman Emperor Constantine the Great in 330 AD. It was the capital of the Byzantine Empire (Western Roman Empire) not Greek. Constantinople was renamed to Istanbul when the Ottoman's conquered the Byzantine Empire. Constantinople fell April 6th, 1453. From that time onward it was a Muslim city controlled by the Turks. First under Ottoman control and finally under secular Turkish control beginning in 1908.
The City never belonged to Greece. Greece was neutral for much of WWI.
The Hellenistic government was overthrown in 1916 and the new government did declared war on the Axis in June of 1916 while it's own civil war was ongoing. The new Greek government did want control of Constantinople after the war but Russia veto'd this.
The Constantinople Agreement
The Greek government was neutral, but in 1915 it negotiated with the Allies, offering soldiers and especially a geographical launching point for attacks on the Straits. Greece itself wanted control of Constantinople. Russia vetoed the Greek proposal, because its main war goal was to control the Straits, and take control of Constantinople.2 The UK and France both agreed, while putting forward their own claims, to an increased sphere of influence in Iran in the case of the UK and to annexation of Syria (including Palestine) and Cilicia for France. The UK and French claims were both agreed all sides also agreeing that the exact governance of the Holy Places was to be left for later settlement.
The Constantinople Agreement were a series of secret assurances in which the allies had agreed to give Constantinople and the Dardanelles to Russia. Although Constantinople was occupied by the Allies at the end of WWI in 1918, Russia had experienced the Communist Revolution, withdrawn from the war and was no longer considered one of the Allied Powers.