information:classification, linguistic:paraphrasing, stylistic:style_transfer, structural:reformatting
Age of Imperialism (1855–1916)

Ethiopian isolationism ended following a British mission that concluded with an alliance between the two nations, but it was not until 1855 that the Amhara kingdoms of northern Ethiopia (Gondar, Gojjam, and Shewa) were briefly united after the power of the emperor was restored beginning with the reign of Tewodros II. Tewodros II began a process of consolidation, centralisation, and state-building that would be continued by succeeding emperors. This process reduced the power of regional rulers, restructured the empire's administration, and created a professional army. These changes created the basis for establishing the effective sovereignty and territorial integrity of the Ethiopian state. In 1875 and 1876, Ottoman and Egyptian forces, accompanied by many European and American advisors, twice invaded Abyssinia but were initially defeated. From 1885 to 1889 (under Yohannes IV), Ethiopia joined the Mahdist War allied to Britain, Turkey, and Egypt against the Sudanese Mahdist State. In 1887, Menelik II, king of Shewa, invaded the Emirate of Harar after his victory at the Battle of Chelenqo. On 10 March 1889, Yohannes IV was killed by the Sudanese Khalifah Abdullah's army whilst leading his army in the Battle of Gallabat.

Ethiopia, in roughly its current form, began under the reign of Menelik II, who was Emperor from 1889 until his death in 1913. From his base in the central province of Shewa, Menelik set out to annex territories to the south, east, and west — areas inhabited by the Oromo, Sidama, Gurage, Welayta, and other peoples. He achieved this with the help of Ras Gobana Dacche 's Shewan Oromo militia, which occupied lands that had not been held since Ahmad ibn Ibrahim al-Ghazi 's war, as well as other areas that had never been under Ethiopian rule.

For his leadership, despite opposition from more traditional elements of society, Menelik II was heralded as a national hero. He had signed the Treaty of Wuchale with Italy in May 1889, by which Italy would recognize Ethiopia's sovereignty so long as Italy could control an area north of Ethiopia (now part of modern Eritrea). In return, Italy was to provide Menelik with weapons and support him as emperor. The Italians used the time between the signing of the treaty and its ratification by the Italian government to expand their territorial claims. This First Italo–Ethiopian War culminated in the Battle of Adwa on 1 March 1896, in which Italy's colonial forces were defeated by the Ethiopians. During this time, about a third of the population died in the Great Ethiopian Famine (1888 to 1892). and the rinderpest swept through the area, destroying much of the herd economy. On 11 October 1897, Ethiopia adopted the colours of the pan-African flag with green, yellow and red stripes in representation of pan-Africanist ideology.