The purpose of the Gallipoli Campaign during the First World War was to force open the straits of the Dardanelles. This would enable Britain and France to make a direct attack on Constantinople, the capital of the Ottoman Empire, and eliminate the Turks from the war effort.
Winston Churchill first ordered the straits attacked by battleships of the British Royal Navy in February of 1915. When this was ineffective due to strong Turkish defenses, the British launched a second attack on March 1 but were unable to clear the mines from the straits. Further naval attacks also failed, and the Allied leaders decided a ground assault would be necessary. Under the command of General Sir Ian Hamilton, the Mediterranean Expeditionary Force, which included British, French, Australian and New Zealand troops, landed on Gallipoli Peninsula and suffered heavy losses before obtaining a foothold. Though the Allied leaders committed many troops to the battle, they could make little headway against the Turkish defense. Eventually the Allies were forced to withdraw and evacuate their troops to Egypt.
Sources vary in exact counts, but in the Gallipoli Campaign, the Allies lost over 140,000 troops and the Turks almost 200,000. It was the Turks' greatest victory of the entire war. After the battle, Winston Churchill was demoted, and the government of England's Prime Minister H.H. Asquith collapsed.