The occupation of Constantinople  (November 13, 1918 – October 4, 1923), the capital of the Ottoman Empire, by British, French and Italian forces, took place in accordance with the Armistice of Mudros, which ended Ottoman participation in the First World War. The first French troops entered the city on November 12, 1918, followed by British troops the next day. The Italian troops landed in Galata on February 7, 1919

Allied troops occupied zones based on the sections of Constantinople (now Istanbul) and set up an Allied military administration early in December 1918. The occupation had two stages: the initial phase in accordance with the Armistice gave way in 1920 to a more formal arrangement under the Treaty of Sèvres. Ultimately, the Treaty of Lausanne, signed on 24 July 1923, led to the end of the occupation. The last troops of the Allies departed from the city on 4 October 1923, and the first troops of the Ankara government, commanded by Şükrü Naili Pasha (3rd Corps), entered the city with a ceremony on 6 October 1923, which has been marked as the Liberation Day of Istanbul (Turkish: İstanbul'un Kurtuluşu) and is commemorated every year on its anniversary.

1918 saw the first time the city had changed hands since the Fall of Constantinople in 1453. The city had been occupied by crusaders in 1204, than by Ottomans in 1453 & finally by Allied forces in 1918 after being defended in Gallipoli war for a long time. Along with the Occupation of Smyrna it mobilized the establishment of the Turkish National Movement and the Turkish War of Independence.

https://en.wikipedia.org/wiki/Occupation_of_Constantinople