The occupation of Istanbul (Turkish: stanbul'un gali; 13 November 1918 4 October 1923), the capital of the Ottoman Empire, by British, French, Italian, and Greek 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 12 November 1918, followed by British troops the next day. The Italian troops landed in Galata on 7 February 1919.Allied troops occupied zones based on the existing divisions of 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 Svres. 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 kr 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: stanbul'un Kurtuluu) 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. Along with the Occupation of Smyrna, it spurred the establishment of the Turkish National Movement, leading to the Turkish War of Independence.

The Allies of World War I or Entente Powers were a coalition of countries led by France, Britain, Russia, Italy, Japan, and the United States against the Central Powers of Germany, Austria-Hungary, the Ottoman Empire, Bulgaria, and their colonies during the First World War (1914–1918).

By the end of the first decade of the 20th century, the major European powers were divided between the Triple Entente and the Triple Alliance. The Triple Entente was made up of France, Britain, and Russia. The Triple Alliance was originally composed of Germany, Austria–Hungary, and Italy, but Italy remained neutral in 1914. As the war progressed, each coalition added new members. Japan joined the Entente in 1914 and after proclaiming its neutrality at the beginning of the war, Italy also joined the Entente in 1915. The term "Allies" became more widely used than "Entente", although the Principal Allies of France, Britain, Russia, Italy, and Japan were sometimes known also as Quintuple Entente. The colonies administered by the countries that fought for the allies were also part of the Entente Powers such as British India, French Indochina, and Japanese Korea.

The United States joined in 1917 (the same year in which Russia withdrew from the conflict) as an "associated power" rather than an official ally. Other "associated members" included Serbia, Belgium, Montenegro, Asir, Nejd and Hasa, Portugal, Romania, Hejaz, Panama, Cuba, Greece, China, Siam (now Thailand), Brazil, Armenia, Guatemala, Nicaragua, Costa Rica, Haiti, Liberia, Bolivia, Ecuador, Uruguay and Honduras. The treaties signed at the Paris Peace Conference recognised Great Britain, France, Italy, Japan and the United States as 'the Principal Allied and Associated Powers'.