Emperor Tewodros II was born Lij Kassa in Qwara, in 1818. His father was a small local chief, and his relative (possible uncle) Dejazmach Kinfu was governor of the provinces of Dembiya, Qwara and Chelga between Lake Tana and the northwestern frontier.
Kassa lost his inheritance upon the death of Kinfu while he was still a young boy. After receiving a traditional education in a local monastery, he went off to lead a band of bandits that roved the country in a Robin Hood-like existence. His exploits became widely known, and his band of followers grew steadily until he led a formidable army. He came to the notice of the ruling Regent, Ras Ali, and his mother Empress Menen Liben Amede (wife of the puppet Emperor Yohannes III). In order to bind him to them, Ras Ali and the Empress arranged for Kassa to marry Ali's daughter, and upon the death of his uncle Kinfu, he was made chief of Kwara and all Dembea with the title of Dejazmatch. He turned his attention to conquering the remaining chief divisions of the country, Gojjam, Tigray and Shewa, which still remained unsubdued. His relations with his father-in-law and grandmother-in-law deteriorated however, and he soon took up arms against them and their vassals, and was successful.
On February 11, 1855, Kassa deposed the last of the Gondarine puppet Emperors, and was crowned negusa nagast of Ethiopia under the name of Tewodros II. He soon after advanced against Shewa with a large army. Chief of the notables opposing him was its king Haile Melekot, a descendant of Meridazmach Asfa Wossen. Dissensions broke out among the Shewans, and after a desperate and futile attack on Tewodros at Dabra Berhan, Haile Melekot died of illness, nominating with his last breath his eleven-year-old son as successor (November 1855) under the name Negus Sahle Maryam (the future emperor Menelek II). Darge, Haile Melekot's brother, and Ato Bezabih, a Shewan noble, took charge of the young prince, but after a hard fight with Angeda, the Shewans were obliged to capitulate. Sahle Maryam was handed over to the Emperor, taken to Gondar, and there trained in Tewodros's service, and then placed in comfortable detention at the fortress of Magdala. Tewodoros afterwards devoted himself to modernizing and centralizing the legal and administrative structure of his kingdom, against the resistance of his governors. Sahle Maryam of Shewa was married to Tewodros II's daughter Alitash.
In 1865, Sahle Maryam escaped from Maqdala, abandoning his wife, and arrived in Shewa, and was there acclaimed as Negus. During Tewodros's reign, the alliance he forged between Britain and Ethiopia was ended in 1868 when the Emperor, angered by Queen Victoria's failure to reply to his diplomatic overtures, imprisoned several British subjects. The British sent an army to free the subjects which defeated the Ethiopians and, in turn, led Tewodros to fall from power and commit suicide on April 13, 1868. On the death of Tewodros, many Shewans, including Ras Darge, were released, and the young Negus of Shewa began to feel himself strong enough, after a few preliminary minor campaigns, to undertake offensive operations against the northern princes. But these projects were of little avail, for Ras Kassai of Tigray, had by this time (1872) risen to supreme power in the north. Proclaiming himself negusa nagast under the name of Yohannes (or John) IV, he forced Sahle Maryam to acknowledge his overlordship.