Why did the Allies land at Gallipoli, and why did they fail?

The Allies landed at Gallipoli to knock the Ottoman Empire out of the war and open a sea route to Russia, and they failed because the Ottoman defenders held the high ground and the plan fell apart on the beaches. Britain, France and Russia tried to make the Ottoman Empire surrender by taking control of the Turkish straits, so that warships could bombard Constantinople and cut it off from the Asian part of the empire. A defeat could also have opened the straits to Allied supplies headed for the Black Sea and warm-water ports in Russia.

The idea began at sea. Winston Churchill, First Lord of the Admiralty, proposed a naval attack on the Dardanelles, based in part on erroneous reports of Ottoman troop strength. On 18 March 1915 French battleships sailed through a line of mines that the Ottoman minelayer Nusret had placed secretly ten days earlier, and the losses forced the fleet to recall. After that, planning began to capture the Ottoman defences by land.

The landings came on 25 April. The ANZAC covering force landed about two kilometres too far north, over steeply rising ground, owing to undetected currents or a navigational error. By mid-morning Mustafa Kemal had reorganised the defenders for a counter-attack. ANZAC casualties on the first day numbered around 2,000 killed or wounded, and the landings were contained in a perimeter less than two kilometres long.

In January 1916, after eight months of fighting, with an estimated 250,000 casualties on each side, the land campaign was abandoned and the invasion force withdrawn, an Ottoman victory.