The year 1977 saw the emergence of the most serious external challenge to the revolutionary regime that had yet materialized. The roots of the conflict lay with Somali irredentism and the desire of the Somali government of Mahammad Siad Barre to annex the Ogaden area of Ethiopia. Somalia's instrument in this process was the Western Somali Liberation Front (WSLF), a Somali guerrilla organization, which by February 1977 had begun to take advantage of the Derg's political problems as well as its troubles in Eritrea to attack government positions throughout the Ogaden (see The Somali, ch. 5). The Somali government provided supplies and logistics support to the WSLF. Through the first half of the year, the WSLF made steady gains, penetrating and capturing large parts of the Ogaden from the Dire Dawa area southward to the Kenya border.

The increasingly intense fighting culminated in a series of actions around Jijiga in September, at which time Ethiopia claimed that Somalia's regular troops, the Somali National Army (SNA), were supporting the WSLF. In response, the Somali government admitted giving "moral, material, and other support" to the WSLF. Following a mutiny of the Ethiopian garrison at Jijiga, the town fell to the WSLF. The Mengistu regime, desperate for help, turned to the Soviet Union, its ties to its former military supplier, the United States, having foundered in the spring over the Derg's poor human rights record. The Soviet Union had been supplying equipment and some advisers for months. When the Soviet Union continued to aid Ethiopia as a way of gaining influence in the country, Somalia, which until then had been a Soviet client, responded by abrogating its Treaty of Friendship and Cooperation with Moscow and by expelling all Soviet advisers.

The Soviet turnaround immediately affected the course of the war. Starting in late November, massive Soviet military assistance began to pour into Ethiopia, with Cuban troops deploying from Angola to assist the Ethiopian units. By the end of the year, 17,000 Cubans had arrived and, with Ethiopian army units, halted the WSLF momentum. On February 13, 1978, Mogadishu dispatched the SNA to assist the WSLF, but the Somali forces were driven back toward the border. After the Ethiopian army recapture of Jijiga in early March, the Somali government decided to withdraw its forces from the Ogaden, leaving the Ethiopian army in control of the region. However, in the process of eliminating the WSLF threat, Addis Ababa had become a military client of Moscow and Havana, a situation that had significant international repercussions and that resulted in a major realignment of power in the Horn of Africa.

Data as of 1991