The Gallipoli campaign was a military campaign in the First World War that took place on the Gallipoli peninsula (Gelibolu in modern Turkey), from 19 February 1915 to 9 January 1916. The Entente powers, Britain, France and the Russian Empire, sought to weaken the Ottoman Empire, one of the Central Powers, by taking control of the Ottoman straits. This would expose the Ottoman capital at Constantinople to bombardment by Allied battleships and cut it off from the Asian part of the empire. With Turkey defeated, the Suez Canal would be safe and a year-round Allied supply route could be opened through the Black Sea to warm-water ports in Russia.

The attempt by the Allied fleet to force a passage through the Dardanelles in February 1915 failed and was followed by an amphibious landing on the Gallipoli peninsula in April 1915. In January 1916, after eight months' fighting, with approximately 250,000 casualties on each side, the land campaign was abandoned and the invasion force withdrawn. It was a costly campaign for the Entente powers and the Ottoman Empire as well as for the sponsors of the expedition, especially the First Lord of the Admiralty (1911–1915), Winston Churchill. The campaign was considered a great Ottoman victory. In Turkey, it is regarded as a defining moment in the history of the state, a final surge in the defence of the motherland as the Ottoman Empire retreated. The struggle formed the basis for the Turkish War of Independence and the declaration of the Republic of Turkey eight years later, with Mustafa Kemal Atatürk, who rose to prominence as a commander at Gallipoli, as founder and president.

The campaign is often considered to be the beginning of Australian and New Zealand national consciousness; 25 April, the anniversary of the landings, is known as Anzac Day, the most significant commemoration of military casualties and veterans in the two countries, surpassing Remembrance Day (Armistice Day).