The Naval Operations in the Dardanelles Campaign (17 February 1915 – 9 January 1916) took place against the Ottoman Empire during the First World War. Ships of the Royal Navy, French Marine nationale, Imperial Russian Navy (Российский императорский флот) and the Royal Australian Navy, attempted to force the defences of the Dardanelles Straits. The straits are a narrow waterway connecting the Mediterranean Sea with the Black Sea, via the Aegean, Sea of Marmara and the Bosphorus. The Dardanelles Campaign began as a naval operation but the success of the Ottoman defence led to the Gallipoli Campaign, an attempt to occupy the Gallipoli peninsula with land forces supported by the navies, to open the sea route to Constantinople. The Allies also tried to pass submarines through the Dardanelles to attack Ottoman shipping in the Sea of Marmara.
The mouth of the strait is 2.3 mi (3.7 km) wide with a rapid current emptying from the Black Sea into the Aegean. The distance from Cape Helles to the Sea of Marmora is about 41 mi (66 km), overlooked by the heights on the Gallipoli peninsula and lower hills on the Asiatic shore. The passage widens for 5 mi (8.0 km) to Eren Keui Bay, the widest point of the strait at 4.5 mi (7.2 km), then narrows for 11 mi (18 km) to Kephez Point, where the waterway is 1.75 mi (2.82 km) wide and then broadens as far as Sari Sighlar Bay. The narrowest part of the strait is 14 mi (23 km) upstream, from Chanak to Kilid Bahr at 1,600 yd (1,500 m), where the channel turns north and widens for 4 mi (6.4 km) to Nagara Point. From the point, the passage turns north-east for the final 23 mi (37 km) to the Sea of Marmora. The Ottomans used the term "fortress" to describe the sea defences of the Dardanelles on both sides of the waterway from the Aegean approaches to Chanak. In 1914 only the defences from the entrance of the straits and 4 mi (6.4 km) from the north end of Kephez Bay to Chanak had been fortified. 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.
At the beginning of the 20th century, the Ottoman Empire had the reputation of being the sick man of Europe, weakened by political instability, military defeat and civil strife, following a century of slow decline. The economic resources of the Ottoman Empire were depleted by the cost of the Balkan Wars of 1912 and 1913 and the French, British and Germans had offered financial aid. A pro-German faction influenced by Enver Pasha, the former Ottoman military attaché in Berlin, opposed the pro-British majority in the Ottoman cabinet and tried to secure closer relations with Germany. In December 1913, the Germans sent a military mission to Constantinople, headed by General Otto Liman von Sanders. The geographical position of the Ottoman Empire meant that Russia, France and Britain had a significant interest in Ottoman neutrality.
During the Sarajevo Crisis in 1914, German diplomats offered Turkey an anti-Russian alliance and territorial gains in Caucasia, north-west Iran and Trans-Caspia. The pro-British faction in the Cabinet was isolated due to the British ambassador taking leave until 18 August. The Ottoman policy was to obtain a guarantee of territorial integrity and potential advantages, unaware that the British might enter a European war. On 30 July 1914, two days after the outbreak of the war in Europe, the Ottoman leaders agreed to form a secret Ottoman-German Alliance against Russia, although it did not require them to undertake military action. On 2 August, the British requisitioned the modern battleships Sultân Osmân-ı Evvel and Reşadiye which British shipyards had been building for the Ottoman Navy, alienating pro-British elements in Constantinople, despite the offer of compensation if the Ottomans remained neutral. The unilateral action strained Ottoman-British diplomatic relations and to gain more influence, the German government offered SMS Goeben and SMS Breslau to the Ottoman navy as replacements. The Allies tried to intercept the ships, which escaped when the Ottoman government opened the Dardanelles to them, despite international law requiring a neutral party to block military shipping. In September, the British naval mission to the Ottomans was recalled and Rear Admiral Wilhelm Souchon of the Imperial German Navy took command of the Ottoman navy. The German naval presence and the success of German armies in Europe, gave the pro-German faction in the Ottoman government enough influence to declare war on Russia.