The Occupation of Constantinople was the occupation of the Ottoman Turkish capital of Constantinople (Istanbul) by the Entente Powers of Britain, France, Italy, Greece, the United States, and Japan following the end of World War I.
On 30 October 1918, the Armistice of Mudros made peace between the Allies and the Ottomans, and it stipulated that the Allies would be allowed to occupy the Bosporus fort and the Dardanelles fort. Admiral Somerset Gough-Calthorpe promised that the British had no intention to dismantle the Ottoman government or militarily occupy Constantinople, and he promised that only British and French troops would be used to occupy the Straits fortifications. On 12 November 1918, a French brigade entered Constantinople, and the first British troops entered the city the following day. In December 1918, the Allies set up a military administration in Constantinople, and an Italian battalion of 19 officers and 740 soldiers landed at the Galata pier on 7 February 1919, to be joined by 283 Carabinieri policemen a day later. On 10 February 1919, the occupation of the old city was allotted to the French, while the British would occupy Pera-Galata and the Italians would occupy Kadikoy and Scutari. Calthorpe had members of Tevfik Pasha's wartime government, high-ranking Ottoman Army soldiers, administrators, and intellectuals arrested and interned on Malta to await trial for war crimes during World War I. The arrests intimidated the Committee of Union and Progress into complying with the occupation forces, and, in accordance with the Ottoman tradition of making up for failures with the "falling of heads", the Sultan allowed the Allies to try several CUP leaders from 28 April 1919 to 29 March 1920, and the trials were conducted in the Western style on Malta rather than in the Ottoman style in Constantinople. Seven defendants were found guilty on 22 July 1919, but the Allies refused to use the ample evidence volunteered by the Ottoman government (which intended to offer the heads of the war criminals to repair relations with the Allies), and all of the "Malta exiles" were released on the orders of John de Robeck.
At the same time, the British occupiers became aware that the nationalist Turkish National Movement had been formed with the objective of freeing Turkey from foreign occupation. Calthorpe forced the Ottomans to strip the post of Inspector General of its powers after discovering that the victor of the Gallipoli campaign and a Turkish national hero, Mustafa Kemal, had been granted that title. Calthorpe's 23 June 1919 report to the Foreign and Commonwealth Office about Kemal's role in the establishment of the National Movement was ignored by the Eastern Department, and, while a British Army captain in Samsun warned Calthorpe about the Turkish nationalists, his soldiers were replaced by Nepalese Gurkhas. On 5 August 1919, Calthorpe was transferred and replaced by John de Robeck, who was appointed "Commander-in-Chief, Mediterranean and High Commissioner at Constantinople". In 1920, De Robeck was warned that the Turks were receiving substantial arms shipments, writing to London to ask against whom the arms would be employed. At the February 1920 Conference of London, De Robeck warned its participants that Anatolia was moving into a resistance stage, and demands for national sovereignty became louder as the United States withdrew into isolationism.
The newly elected nationalist Ottoman Parliament did not recognize the Allied occupation and developed a "National Pact", which called for self-determination, the security of Constantinople, the opening of the Straits, and the abolition of the capitulations. The Khilafat Movement in India also demanded that the British Empire preserve the existence of the Ottoman caliphate. As the Treaty of Sevres was negotiated in London, the Allies decided to partition Ottoman between Greek, French-Armenian, Italian, and Armenian administrations, but Muslim citizens of the empire saw the partition plan as depriving them of sovereignty. Muslim unrest across Anatolia persuaded Britain that military administration was the only way to protect the Ottoman Empire's Christian subjects, and that the Turkish National Movement needed to be suppressed.
On 15 March 1920, British troops began to occupy key buildings in Constantinople and arrest Turkish nationalists, but the 10th Division and the military music school resisted arrest, and at least 10 students were killed by the British Indian Army. On 18 March 1920, the Ottoman Parliament issued a formal protest against the Allied occupation authorities, leading to the British dissolving the Ottoman Parliament and leaving the Sultan as the sole controller of the Empire, and a British puppet. General Sir George Milne announced that the Allies had no intention of taking over the government, that the Allies sought to keep the Straits open and protect the Armenians, persuaded the Ottoman government to denounce the Turkish nationalists and send many into exile, and Damat Ferid Pasha was appointed to head a liberal and pro-British government in Constantinople.
The British decided that the nationalist insurgency in Anatolia should be suppressed by local forces with the help of British training and arms, and the British supported "independent armies" in central Anatolia in fighting against the nationalists. These forces failed to suppress the nationalists, and Milne's request for reinforcement by 27 divisions was rejected, as the British government would not be able to handle the political consequences of such a deployment. Mustafa Kemal soon established a nationalist opposition government in Ankara, challenging the pro-British Ottoman government in Constantinople. On 18 October 1920, Ahmed Tevfik Pasha was appointed to replace Damat Ferid Pasha as head of government, and his attempt to persuade Kemal to let him recall the Ottoman Senate to ratify the Treaty of Sevres was unsuccessful, instead resulting in a nationalist offensive.
The success of the Turkish forces against the French and Greeks led to the British deciding to resist any attempts to penetrate the neutral zone of the Straits, and the 1922 Chanak Crisis nearly led to open war between Britain and Turkey, and also led to the British signing the Armistice of Mudanya with Turkey on 11 October 1922. The British response to the crisis led to the downfall of Prime Minister David Lloyd George's coalition government, forever ending the Liberal Party's rule over Britain. On 1 November 1922, the Turkish National Movement abolished the Ottoman Sultanate, and, on 1 November 1922, Sultan Mehmed VI was expelled from Constantinople. On 20 November 1922, peace talks between the Allies and Turkey began at the Conference of Lausanne, and, on 24 July 1923, the Treaty of Lausanne led to the Allies withdrawing from Constantinople from 23 August to 4 October 1923. On 6 October 1923, the Turkish forces loyal to Kemal's Ankara government triumphantly entered into Constantinople, and, on 29 October 1923, the Grand National Assembly proclaimed the establishment of the Republic of Turkey, with Ankara as its capital and Kemal as its first President.