At the outbreak of the war Turkey, though angry about British refusing to hand over two newly built battleships, was determined to remain neutral. The events quickly overtook Turkish plans. When the war broke out German battlecruiser SMS Goeben and light cruiser SMS Brealau were trapped in the Mediterranean Sea. With Royal Navy in hot pursuit, the ships sailed to Turkey, where German diplomats offered to hand over the ships to the Turks. The ships were then commissioned in the Ottoman Navy as Yavuz Sultan Selim and Midilli, but retained their German crews. Taking orders from Berlin instead of Constantinople, the two ships then transitioned to the Black Sea (anchoring briefly in front of Russian Ambassador's residence in Constantinople to sing the German National Anthem) to shell Russian ports, thus dragging Turkey into the war on the side of Germany.
In February 1915 combined British and French naval task force attempted to force its way through Dardanelles Straights, sail onto Constantinople and force Turkish surrender. Of the over 100 guns in several coastal batteries along the straights only 14, located at the Narrows, were heavy enough and had enough range to threaten the fleet. These guns protected the minefields that barred passage through the Straights. For a month the capital ships engaged in artillery duels with shore defenses while the British minesweepers attempted to clear the mines. By middle of March 1915 the Turks were nearly out of ammunition and there was a panic in Constantinople as there was every expectation that in just a few days the city would be staring at the muzzles of the British and French guns.
On the morning of March 18 the task force engaged the Turkish defenses south of town of Canakkale once again. And then, as far as the Turks were concerned, a miracle happened. Around 2 o'clock in the afternoon, when the Turkish artillery was largely silenced the French pre-dreadnought battleship Bouvet struck a mine, capsized and sunk in a couple of minutes with a loss of most of her crew. Over the next four hours two British battleships also struck mines and sunk. Two French battleships and a British battlecruiser were also damaged. Despite the losses and with Turkish artillery silenced the British minesweepers could clear the minefields unmolested. The fleet still possessed enough strength, with more reinforcements on the way, to defeat the Ottoman fleet, should it choose to give battle, but British Admiral DeRobeck, the commander of the combined task force, against the advice of his staff officers to press on the attack decided to withdraw, thus snatching defeat from the jaws of victory.
While the ships were busy reducing one by one the Turkish coastal defenses along the Dardanelles, a regiments of Royal Marines made up from ships' Marine detachments actually landed on Gallipoli. The Marines found the peninsula undefended and easily scaled the heights overlooking the Dardanelles. However, the Marines didn't have enough troops to hold their positions and withdrew the next day. This landing served no military purpose at the time but it did show the need for a ground element to compliment the naval operations. It also showed how vulnerable the Turkish shore batteries on the European side of the Straights were to an attack from land, a flaw the Turks rushed to correct.
By April 1915 Turks rushed German-supplied heavy artillery to Gallipoli and deployed two infantry divisions in the Cape Helles area, two more at the north end of peninsula. More troops were stationed on the Asian side of the Straights. In total, Turks deployed over 60,000 men to protect the Dardanelles, backed by German and Austrian advisers and artillery batteries. These troops were well dug in on the high ground, with barbed wire and minefield protecting their positions.
The nearest large British force available for land operation was the Australian and New Zealand Army Corps, (ANZAC) then undergoing training in Egypt for the war in Europe. Indian, British Territorial and French Colonial troops and the British Naval Division also participated in operation.
The main landings at V and W Beaches at Cape Helles led to heavy casualties on both sides. Of the first 200 men of the Royal Munster Fusiliers landing below the old fortress of Seddülbahir from the converted collier River Clyde only 21 made to to the beach. Meantime, the Turkish 57th Infantry regiment that defended the heights above Cape Helles was wiped out to a man. To this day, there is no 57th Regiment in the Turkish Army.
Elsewhere the landings fared better. At Y Beach the the allies landed unopposed and advanced inland, beyond their first day objectives, reaching the Village of Krithia that was only lightly defended at the time. The village controlled the high ground above Cape Helles. Here, the British military tradition of discouraging initiative and only acting on orders reared its ugly head. Lacking orders and unwilling to act on their own initiative the British withdrew. Thousands of men would die in the days to come in futile attempts to capture the village.
Further north at ANZAC Cove the 1st Australian and New Zealand and Australian Divisions landed on the same beach and their units became entangled. It took time to sort out the mess, and four hours after the first elements came to shore the troops started pushing inland. By this time it was too late. While the landing encountered little opposition, by the time the ANZACS attempted to expand the beachhead the Turks moved strong forces to oppose them.
The landing troops were further hampered by a steeply raising unfamiliar terrain crisscrossed by a maze of gullies and lack of accurate maps and absence of howitzers to bombard Turkish positions on the reverse slopes of ridges. The allies relied on ships guns to provide the artillery support but the ships' guns were designed to fire on other ships. They couldn't elevate their barrels to fire on Turkish positions on the high ground above the beachheads. Within days the Gallipoli campaign turned into a bloody stalemate, like the one on the Western Front.