The 1915 Gallipoli Campaign failed because British forces were unable to establish control of the Dardanelle Straits. The primary reasons for the defeat were the failed sea and land attacks as the result of faulty intelligence and fierce Turkish resistance.
The British attack was in response to a Russian appeal for aid in fighting the Turks as the British realized that establishing a shipping route in the Black Sea would make it easier for them to eliminate Turkey from the war. Led by Winston Churchill, first sea lord of the British Admiralty, the bombardment began on Feb. 19 and initially pushed the Turks back from their land positions. However, British intelligence failed to detect a Turkish minefield and, upon re-entering the straits on March 18, the British suffered the loss of three ships and severe damage to three more.
The British land invasion was supported by forces from Australia and New Zealand and established three beach holds. However a lack of knowledge about both the terrain and the strength of Turkish forces meant the allied forces were unable to advance their position. In October, they sustained heavy losses as Turkish reinforcements arrived. By December, British military commanders admitted defeat and began an evacuation.