The British vision of the First World War is dominated by the Western Front, but the war was fought on a wider scale.
On the Eastern Front, Russia fought Germany and Austria-Hungary, generally failing against the former, but fatally weakening the latter before collapsing into revolution in 1917.
Serbia resisted Austria-Hungary until late 1915, when she was overcome by a joint Central Powers invasion. An Anglo-French force despatched to Salonika arrived too late to help.
Italy and Romania joined the Allies in return for promises of Austro-Hungarian territory. The Italians mounted a series of offensives, but eventually required British and French aid themselves. Russia was similarly forced to prop up Romania.
Germany formed an alliance with the Ottoman Empire hoping to attack the British Empire by threatening the Suez Canal and India. The Turks beat off an Allied attempt to force the Dardanelles at Gallipoli but proved unable to sustain any effective offensives. By 1918 the Ottoman Empire was in a state of collapse, having lost Persia and its Arab lands to British invasion.