1916年海尔·塞拉西被封为摄政王。1928年登基为国王。1930年11月2日加冕为海尔·塞拉西一世皇帝。1936年，意大利再次入侵，占领亚的斯亚贝巴，征服埃塞俄比亚全国，塞拉西流亡英国伦敦。1941年盟军击败意大利，同年5月5日塞拉西一世归国复位。1974年国内政变后下台。
Upon the death of Empress Zauditu in 1930, Ras Tafari Makonnen, adopting the throne name Haile Selassie, was crowned Emperor Haile Selassie I of Ethiopia. His full title was “His Imperial Majesty Haile Selassie I, Conquering Lion of the Tribe of Judah, King of Kings of Ethiopia and Elect of God.”
Following the death of Abba Jifar II of Jimma, Emperor Selassie seized the opportunity to annex Jimma. In 1932, the Kingdom of Jimma was formally absorbed into Ethiopia. During the reorganization of the provinces in 1942, Jimma vanished into Kaffa Province.
Emperor Haile Selassie's reign was interrupted in 1935 when Italian forces invaded and occupied Ethiopia.
Emperor Selassie was forced into exile in England despite his plea to the League of Nations for intervention against the Italians.
In spring 1941 the Italians were defeated by British and Allied forces. On May 5, 1941, Emperor Selassie re-enterred Addis Ababa and returned to his throne. The Italians, after their final stand at Gondar in November 1941, conducted a guerrilla war in Ethiopia, that lasted until summer 1943.
After World War II, Emperor Selassie exerted numerous efforts to promote the modernization of his nation. The country's first important school of higher education, University College of Addis Ababa, was founded in 1950. The Constitution of 1931 was replaced with a new one in 1955. The new constitution expanded the powers of the Parliament. While improving diplomatic ties with the United States, Emperor Selassie also sought to improve the nation's relationship with other African nations. To do this, in 1963, he helped to found the Organisation of African Unity.
By the early 1970s, despite his best efforts, Emperor Selassie's advanced age was becoming a major problem for the future of his nation. As Paul B. Henze explains: "Most Ethiopians thought in terms of personalities, not ideology, and out of long habit still looked to Haile Selassie as the initiator of change, the source of status and privilege, and the arbiter of demands for resources and attention among competing groups." Ethiopians worried for their future following his impending death. They worried too whether Emperor Selassie's successors would continue his campaigns for modernization and economic development. However, Henze's perspective can be disputed since the Emperor was not the only source of change, and many times, his views were challenged either by his council or regional leaders. People still protested his 'flight' from Ethiopia during the 1935-1936 Italian invasion, which -he declared- was necessary for him to leave the country to round up allies rather than remain in the country and fight a seemingly impossible war. Many perceived that as fleeing, probably because the western media broadcasted his absence from the country as such.
The Negus was even accused of the expensive war with Eritrean rebels, because he dismissed the federation status of Eritrea and "assimilated" it to Ethiopia in the early 1960s. Indeed, in 1961 the 30-year Eritrean Struggle for Independence began, following the Ethiopian Emperor Haile Selassie I's dissolution of the federation and shutting down of Eritrea's parliament. The Emperor declared Eritrea the fourteenth province of Ethiopia in 1962.
Those accusations, related mainly to the Eritrea independence, were (with other reasons) at the root of the growing marxist movement inside the "Intelligentsia" of Ethiopia. In the early 1970s, the Ethiopian communists received the support of the Soviet Union worldwide expansion during the Leonid Breznev leadership. This help lead to the 1974 marxist coup of Mengistu, supported even by problems related to land reforms.
The government's failure to effect significant economic and political reforms over the previous fourteen years—combined with rising inflation, corruption, a famine that affected several provinces (especially Welo and Tigray) and that was concealed from the outside world, and the growing discontent of urban interest groups—provided the backdrop against which the Ethiopian revolution began to unfold in early 1974.
Whereas elements of the urban-based, modernizing elite previously had sought to establish a parliamentary democracy, the initiation of the 1974 revolution was the work of the military, acting essentially in its own immediate interests. The unrest that began in January of that year then spread to the civilian population in an outburst of general discontent .
After a period of civil unrest which began in February 1974, the aging Emperor Haile Selassie I was deposed. On September 12, 1974, a provisional administrative council of soldiers, known as the Derg ("committee") seized power from the emperor and installed a government which was socialist in name and military in style. The Derg summarily executed 59 members of the former government, including two former Prime Ministers and Crown Councilors, Court officials, ministers, and generals. Emperor Selassie died on August 22, 1975. He was allegedly strangled in the basement of his palace.