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Rise to power

Kassa Hailu was born into a country rife with civil war, and he defeated many regional noblemen and princes before becoming emperor during time known as the Zemene Mesafint or "Age of the Princes". During this era, regional princes, and noble lords of diverse ethnic and religious backgrounds vied with each other for power and control of the Gondarine Emperor. A puppet Emperor of the Solomonic dynasty was enthroned in Gondar by one nobleman, only to be dethroned and replaced by another member of the Imperial dynasty when a different regional prince was able to seize Gondar and the reins of power. Regions such as Gojjam and Shewa were ruled by their own branches of the Imperial dynasty and, in Shewa, the local prince went as far as assuming the title of King. In Wollo, competing royal powerful Oromo and Muslim dynasties also vied for power. Nevertheless, a semblance of order and unity was maintained in northern Ethiopia during the era of the Princes by the powerful Rases of the Were Sheik dynasty of Wollo such as Ras Ali the Great and Ras Gugsa who controlled Gondar and the Emperor.

Kassa began his career in this era as a shifta (outlaw), but after amassing a sizable force of followers, was able to not only restore himself to his father's previous fiefdom of Qwara but was able to control all of Dembiya. Moreover, he gained popular support by his benevolent treatment of the inhabitants in the areas he controlled: according to Sven Rubenson, Kassa "shared out captured grain and money to the peasants in Qwara and told them to buy hoes and plant." This garnered notice of the nobleman in control of Gondar, Ras Ali II of Yejju of Wollo. Empress Menen Liben Amede, wife of Emperor Yohannes III, and the mother of Ras Ali, arranged for Kassa to marry her granddaughter, Tewabech Ali. She awarded him all of Ye Meru Qemas in the hopes of binding him firmly to her son and herself.

Although all sources and authorities believe that Kassa truly loved and respected his wife, his relationship with his new in-laws deteriorated largely because of the disdainful treatment he repeatedly received from the Empress Menen. By 1852, he rebelled against Ras Ali and, in a series of victories – Gur Amaba, Takusa, Ayshal, and Amba Jebelli – over the next three years he defeated every army the Ras and the Empress sent against him. At Ayshal he captured the Empress Menen, and Ras Ali fled. Kassa announced that he was deposing Emperor Yohannes III, and then marched on his greatest remaining rival, Dejazmach Wube Haile Maryam of Semien. Kassa refused to acknowledge an attempt to restore the former Emperor Sahle Dengel in the place of the hapless Yohannes III who had acknowledged Kassa immediately. Yohannes III was treated well by Kassa who seems to have had some personal sympathy for him. His views on Sahle Dengel are not known but are not likely to have been sympathetic. Following the defeat of Dejazmach Wube, Kassa was crowned Emperor by Abuna Salama III in the church of Derasge Maryam on February 11, 1855. He took the throne name of Tewodros II, attempting to fulfill a prophecy that a man named Tewodros would restore the Ethiopian Empire to greatness and rule for 40 years.