Why did the allies decide to attack Gallipoli in WW1?

The allies decided to attack Gallipoli on April 15th, 1915 largely by the urging of Sir Winston Churchill who saw it as a way to end the stalemate on the western front. British strategists had also for many years believed that the best defense of Egypt and the Suez Canal was an attack on Turkey. If the campaign succeeded it would be a decisive blow to the Germans and give the upper hand to the Allies.

The Russians on the western front were slowly falling apart after the Germans had delivered a crushing blow at Tannenberg. A Turkish advance was also threatening the Russians and so it appealed to its allies for assistance. If the allies could gain control of the Dardanelles not only would this re-establish communications with Russia, but it would release 350,000 tons of wheat and shipping locked in the Black Sea by Turkey. The Black Sea would then become an open trade route to enable the allies to transport supplies to Russia in return for grain.

If a victory in Gallipoli succeeded, the allies predicted that Turkey would surrender, thus removing an important German ally. This would then lower German morale and boost allied morale. It would also probably lead to the Greeks, Italians, Bulgarians and the Romanians joining the allies against Germany.

With the control of the Dardanelles, the allies would be able to open a southern front in Germany, causing it to shift some of the forces on the bloody western front and relieve the pressure on Russia. The allies predicted that by doing this, it would stretch the German army and lead to an earlier end to the war.

The Gallipoli campaign was a good plan in theory but because of inaccurate intelligence the attacks never succeeded. If it succeeded as planned, the benefits of it would give the allies an enormous advantage over the Germans.