On November 2, 1914, Russia declared war on the Ottoman Empire, four days after Ottoman warships commanded by German officers had shelled the Russian ports of Odessa and Sevastopol and Novorossiysk on the Black Sea. Britain and France declared war on the Ottoman state three days later. The empire had signed a secret alliance with Germany in August, and the attack on the ports was meant to push the country into the war on the German side. It worked.
The Ottoman entry closed the straits between the Black Sea and the Mediterranean. That cut the route by which Russia sent grain and received munitions from Britain and France, and it left the Russian armies short of shells and rifles, and the shortage was felt in the field within months. The straits mattered. The allies looked for another way to reach Russia and to knock the Ottoman state out of the war, and in 1915 they attacked the Dardanelles and landed at Gallipoli, where the campaign failed with heavy losses on both sides.
The war also opened fronts far from Europe. Russian and Ottoman armies fought in the Caucasus, and the Ottoman advance in the winter of 1914 into the mountains ended in disaster at Sarikamish, and British and Indian forces landed in Mesopotamia and moved up the rivers toward Baghdad, and an Ottoman force reached the Suez Canal, and Arab groups were drawn into the fighting by promises of a state of their own. The fronts multiplied.
The demands of the war broke the empire. The army lost hundreds of thousands of men, the economy came under foreign control through a debt administration imposed before the war, and the government deported and killed large numbers of Armenians in 1915 and 1916 in a campaign that is now described as genocide. The state fell in 1918. The peace that followed took its Arab provinces and left a smaller republic in Anatolia.