World War I
A war fought between the Allied Powers – Britain, France, Russia, Japan, and Serbia – who were joined in the course of the war by Italy (1915), Portugal and Romania (1916), the USA and Greece (1917) – against the Central Powers: Germany, the Austro‐Hungarian empire, Ottoman Turkey, and Bulgaria (from 1915). The war's two principal causes were fear of Germany's colonial ambitions and European tensions arising from shifting diplomatic divisions and nationalist agitation, especially in the Balkans. It was fought in six main theatres of war. On the Western Front fighting was characterized by trench warfare, both sides believing that superiority in numbers would ultimately prevail despite the greater power of mechanized defence. Aerial warfare developed from reconnaissance into bombing and the use of fighter aircraft in air‐to‐air combat. On the Eastern Front the initial Russian advance was defeated at Tannenberg (1914). With Turkey also attacking Russia, the Dardanelles expedition (1915) was planned in order to provide relief, but it failed. Temporary Russian success against Austria‐Hungary was followed (1917) by military disaster and the Russian Revolution. The Mesopotamian Campaign was prompted by Britain's desire to protect oil installations and to conquer outlying parts of the Ottoman empire. A British advance in 1917 against the Turks in Palestine, aided by an Arab revolt, succeeded. In north‐east Italy a long and disastrous campaign after Italy had joined the Allies was waged against Austria‐Hungary, with success only coming late in 1918. Campaigns against Germany's colonial possessions in Africa and the Pacific were less demanding. At sea there was only one major encounter, the inconclusive Battle of Jutland (1916). A conservative estimate of casualties of the war gives 10 million killed and 20 million wounded. An armistice was signed and peace terms agreed in the Versailles Peace Settlement.