On November 5, 1914, Britain and France declared war on the Ottoman Empire, three days after Russia had done so and six days after Ottoman warships commanded by German officers had shelled Russian ports on the Black Sea. The empire had signed a secret alliance with Germany in August, and it had closed the straits between the Black Sea and the Mediterranean a month later, and the attack on the Russian ports was the step that turned a neutral state into a belligerent. The step was planned in Berlin.
The arrival of the empire on the German side closed the sea route by which Russia sent grain abroad and received munitions from its allies, and the closure left the Russian armies short of shells and rifles through 1915. The effect was severe. The allies looked for a way to reopen the route and to knock the Ottoman state out of the war, and in 1915 they attacked the Dardanelles by sea and then landed on the Gallipoli peninsula, where the campaign failed with heavy losses on both sides.
The fighting spread through the region. Russian and Ottoman armies met in the Caucasus, and the Ottoman advance into the mountains in the winter of 1914 ended in disaster at Sarikamish, and British and Indian forces landed in Mesopotamia and moved up the rivers toward Baghdad, and an Ottoman column crossed the desert toward the Suez Canal, and Arab groups were drawn in by promises of a state of their own. The war spread.
The demands of the war broke the empire. The army lost hundreds of thousands of men, the state's finances came under the control of its creditors, and the government deported and killed large numbers of Armenians in 1915 and 1916 in a campaign that is now described as genocide. The empire surrendered in 1918. The settlement that followed took its Arab provinces and left a smaller republic in Anatolia, and the state that had held the eastern Mediterranean for four centuries was gone.