Why were the Anzacs sent to Gallipoli?
The German army had delivered a crushing blow to Russia at Tannenberg at the start of the war and had been driving eastwards. The Russians were threatened by a Turkish advance through the Caucasus and appealed to their allies for assistance. Gaining control of the Dardanelles would re-establish communications with Russia and release wheat and shipping locked in the Black Sea by Turkey.
Besides this, British strategists had for many years before the war believed that the best defence of Egypt and the Suez Canal was an attack on Turkey.
The British Royal Navy could have gone a long way towards achieving these goals by steaming through the Dardanelles straits in November 1914 and shelling Constantinople (now Istanbul) and perhaps putting the government to flight. Instead, they cautiously tested the range of the Turkish guns by bombarding the shore batteries.
The Turkish commanders immediately became aware of their vulnerability to further attacks and strengthened their defenses to include carefully laid minefields, well-sited guns and searchlights that swept the narrows at night.
Three months later, a British and French fleet that included 18 battleships, attempted to force its way through to Constantinople. Three capital ships were lost and three crippled.
Unknown to the Allies, the Turkish gun batteries had almost exhausted their ammunition supplies in this effort, and the fleet could have sailed on through the straits with little further damage. Instead, the naval commanders came to the conclusion that they could not force their way through the Dardanelles unless troops were first sent to occupy the Gallipoli Peninsula in force to silence the Turkish guns. Planning for the landing of troops on Gallipoli commenced.