In 1915, Churchill was the First Lord of the Admiralty, the government minister with responsibility for the British navy. In response to a request from the Russian government, the cabinet (the senior policy committee of British government ministers) agreed to support his plan for an attack on the straits of the Dardanelles, in Turkey.
Control of the Dardanelles would allow the allies to get supplies to Russia's Black Sea ports; it was also hoped that the attack would lead to an assault on the Turkish capital, Constantinople, and so force Turkey out of the war and attract new allies in the Balkans.
The original plan was for a naval attack, but this was foiled by Turkish mines. After considerable discussion, and against Churchill's wishes, the decision was taken to abandon the naval campaign and launch a land assault on the Gallipoli peninsula, at the entrance to the Dardanelles straits. By the time it was launched in April 1915, there was already a French presence at Kum Kale. But the Turks were better prepared for defence, and the attack was pinned down on the rocky coast. Despite further landings further up the coast at Suvla Bay in August 1915, the allies were unable to push inland. By the end of the year the allies were losing men heavily for no conceivable advantage, and the decision was taken to evacuate. The evacuation, which took place between 18-19 December 1915, as one of the most successfully planned aspects of the whole campaign and took the Turks completely by surprise.
The failure of the campaign sparked a lot of criticism, much of it directed at Churchill as First Lord of the Admiralty. He was removed from this office in May, and given the token post of Chancellor of the Duchy of Lancaster (a ministerial position without responsibility for a Government department). In November he resigned from the Government and became an officer commanding a battalion on the Western Front.
Criticism of allied planning in the Dardanelles has been particularly acute in Australia and New Zealand, whose forces were included in the allied army.
For definitions of Government ranks and titles, remember to use the glossary section.