The distance from Cape Helles to the Sea of Marmara is about 41 mi (66 km), overlooked by the heights on the Gallipoli peninsula and lower hills on the Asiatic shore.

Until late October 1914, the nature of the seaward defences of the Dardanelles was known to the British and French but after hostilities commenced, information on improvements to the Ottoman fortifications became harder to obtain.

[4] During the Sarajevo Crisis in 1914, German diplomats offered Turkey an anti-Russian alliance and territorial gains, when the pro-British faction in the Cabinet was isolated, due to the British ambassador's absence.

[citation needed] A British naval squadron bombarded the Dardanelles outer defensive forts at Kum Kale and Seddulbahir; a shell hit a magazine and the explosion knocked the guns off their mounts and killed 86 soldiers.

[20] By late 1914, static warfare had begun on the Western Front, with no prospect of a quick decisive victory and the Central Powers had closed the overland trade routes between Britain, France and Russia.

[23] On 2 January 1915, Grand Duke Nicholas of Russia appealed to Britain for assistance against the Ottoman Erzurum Offensive in the Caucasus and planning began for a naval demonstration in the Dardanelles, as a diversion.

[25] On 11 January 1915, the commander of the British Mediterranean Squadron, Vice Admiral S. H. Carden proposed a plan for forcing the Dardanelles using battleships, submarines and minesweepers.

Not only were the majority of the guns of old pattern, with a slow rate of fire and short range, but their ammunition supply was also limited.The Germans secured the appointment of Lieutenant-General Erich Weber as an advisor to the Ottoman GHQ and at the end of August 1914, Vice-Admiral Guido von Usedom, several specialists and 500 men were sent to reinforce the forts on the Dardanelles and Bosphorus.

In September, Usedom was made Inspector-General of Coast Defences and Mines and Vice-Admiral Johannes Merten relieved Weber at Chanak with a marine detachment to operate the modern guns.

Plans were made to build more defensive works in the Intermediate Zone and to bring in mobile howitzers and quick-firers dismounted from older Ottoman ships.

Several heavy howitzers arrived in October but the poor standard of training of the Ottoman gunners, obsolete armaments and the chronic ammunition shortage, which Usedom reported was sufficient only to defend against one serious attack, led him to base the defence of the straits on minefields.

The short bombardment had been extraordinarily successful, destroying the forts at Sedd el Bahr with two shots, that exploded the magazine and dismounted the guns.

The battlecruisers of the Mediterranean Squadron, HMS Indomitable and Indefatigable and the obsolete French battleships Suffren and Vérité, attacked before a formal declaration of war had been made by Britain against the Ottoman Empire.

The strong current in the straits further hampered minesweeping and strengthened Ottoman resolve which had wavered at the start of the offensive; on 4 March, twenty-three marines were killed raiding the outer defences.

A gunnery officer noted in his diary that de Robeck had already expressed misgivings about silencing the Ottoman guns by naval bombardment and that this view was widely held on board the ship.

[44] At 13:54, Bouvet—having made a turn to starboard into Eren Köy Bay—struck a mine, capsized and sank within a couple of minutes, killing 639 crewmen, only 48 survivors being rescued.

Around 16:00, Inflexible began to withdraw and struck a mine near where Bouvet had sunk, thirty crew being killed and the ship taking on with 1,600 long tons (1,600 t) of water.

The abandoned battleships were still floating when the British withdrew but when a destroyer commanded by Commodore Roger Keyes returned to tow or sink the vessels, they could not be found despite a 4-hour search.

For five hours the [destroyer] Wear and picket boats had experienced, quite unperturbed and without any loss, a far more intense fire from them than the sweepers encountered... the latter could not be induced to face it, and sweep ahead of the ships in 'B' line....

[55][56] To place the losses into perspective, the Navy had ordered six hundred new ships during the period Admiral Fisher was First Sea Lord, approximately corresponding with the length of the Dardanelles campaign.

[53]The fleet lost more ships than the Royal Navy had suffered since the Battle of Trafalgar; on 23 March, de Robeck telegraphed to the Admiralty that land forces were needed.

He later told the Dardanelles Commission investigating the campaign, that his main reason for changing his mind, was concern for what might happen in the event of success, that the fleet might find itself at Constantinople or on the Marmara sea fighting an enemy which did not simply surrender as the plan assumed, without any troops to secure captured territory.

[58] With the failure of the naval assault, the idea that land forces could advance around the backs of the Dardanelles forts and capture Constantinople gained support as an alternative and on 25 April, the Gallipoli campaign commenced.

This disagreement contributed to the final resignation of Fisher, followed by the need for Asquith to seek coalition partners to shore up his government and the consequential dismissal of Churchill also.

Kitchener made a proposal to take the Isthmus of Bulair using forty thousand men to allow British ships operating in the Marmara Sea to be supplied across land from the Gulf of Xeros.

On 13 December, the submarine HMS B11 (Lieutenant-Commander Norman Holbrook) had entered the straits, avoiding five lines of mines and torpedoed the Ottoman battleship Mesûdiye, built in 1874, which was anchored as a floating fort in Sari Sighlar Bay, south of Çanakkale.

On 17 April, the British submarine HMS E15 attempted to pass the straits but having dived too deep, was caught in a current and ran aground near Kepez Point, the southern tip of Sarı Sıĝlar Bay, under the guns of the Dardanos battery.

[74] The first French submarine to enter the Sea of Marmara was Turquoise but it was forced to turn back and on 30 October, when returning through the straits, ran aground beneath a fort and was captured intact.

Queen Elizabeth, stationed off Gaba Tepe, had fired across the peninsula at a range of over ten mi (8.7 nmi; 16 km), and sank the transport with the third shot.

[86] With the exception of the continued activity of Allied submarines in the Dardanelles and Sea of Marmara, the only significant naval loss after May was the Laforey-class destroyer HMS Louis which ran aground off Suvla during a gale on 31 October and was wrecked.