(April 25, 1915–Jan. 8, 1916), major land and sea operation of World War I, in which British, French, Australian, and New Zealand forces unsuccessfully attempted an invasion of Turkey. The action was confined to the Dardanelles Strait and the tip of the Gallipoli (Gelibolu) Peninsula near Istanbul.
British casualties were 205,000 out of 410,000; the French sustained a rate of 47,000 out of 79,000; Turkish, 250,000 to 300,000 out of 500,000. The fiasco badly stained the reputations of Churchill, Hamilton, and Kitchener. Despite its overall failure, however, the Gallipoli campaign weakened the Turks enough to facilitate the British seizure of Palestine in 1917. The action also distracted the Germans from a plan they had in 1915 to begin another offensive in France.