The Ogaden War, also known as the Ethio-Somali War (Somali: Dagaalkii Xoraynta Soomaali Galbeed, Amharic: የኢትዮጵያ ሶማሊያ ጦርነት, romanized: ye’ītiyop’iya somalīya t’orinet), was a military conflict between Somalia and Ethiopia fought from July 1977 to March 1978 over control of the sovereignty of the Ogaden region. Somalia launched an invasion in support of the Western Somali Liberation Front (WSLF) insurgency, triggering a broader inter-state war. The intervention drew the disapproval of the Soviet Union, which subsequently withdrew its support for Somalia and backed Ethiopia instead.

Ethiopia was saved from defeat and permanent loss of territory through a massive airlift of military supplies worth $1 billion, the arrival of more than 12,000 Cuban soldiers and airmen and 1,500 Soviet advisors, led by General Vasily Petrov. On 23 January 1978, Cuban armored brigades inflicted the worst losses the Somali forces had ever taken in a single action since the start of the war.

The Ethiopian-Cuban force (equipped with 300 tanks, 156 artillery pieces and 46 combat aircraft) prevailed at Harar and Jijiga, and began to push the Somalis systematically out of the Ogaden. On 23 March 1978, the Ethiopian government declared that the last border post had been regained, thus ending the war. Almost a third of the regular SNA soldiers, three-eighths of the armored units and half of the Somali Air Force had been lost during the war. The war left Somalia with a disorganized and demoralized army as well as a heavy disapproval from its population. These conditions led to a revolt in the army which eventually spiraled into the ongoing Somali Civil War.

The Ogaden is a vast plateau that is overwhelmingly inhabited by Somali people. It represents the westernmost region inhabited by the Somalis in the Horn of Africa, and is located to the south and southeast of the Ethiopian Highlands. During the pre-colonial era the Ogaden region was neither under Ethiopian rule, nor terra nullius, as it was occupied by organized Somali communities. Independent historical accounts are unanimous that previous to the penetration into the region in the late 1880s, Somali clans were free from the control of Ethiopian Empire.

During the late 1880s, as European colonial powers advanced into the Horn of Africa, Ethiopian Emperor Menelik II launched invasions into Somali-inhabited territories as part of his efforts to expand the Ethiopian Empire. These encroachments primarily affected the Ogaden region. The Ethiopian Empire imported a significant amount of firearms from European powers in this period, and the large scale importation of European arms completely upset the balance of power between the Somalis and the Ethiopian Empire, as the colonial powers blocked Somalis from accessing firearms. Control over the Ogaden was expressed through intermittent raids and expeditions that aimed to seize Somali livestock as tribute. In exchange for commercial privileges for British merchants in Ethiopia and the neutrality of Menelik II in the Mahdist War, the British signed the Anglo-Ethiopian treaty of 1897. The agreement ceded large parts of Somali territory to Ethiopia, despite being a legal breach of the legal obligation made by the protectorate.

As indiscriminate raiding and attacks by imperial forces against the Somalis grew between 1890 and 1899, those residing in the plains around the settlement of Jigjiga were in particular targeted. The escalating frequency and violence of the raids resulted in Somalis consolidating behind the anti-colonial Dervish Movement under the lead of Sayyid Mohamed Abdullah Hassan. The Ethiopian hold on Ogaden at the start of the 20th century was tenuous, and administration in the region was "sketchy in the extreme". Sporadic tax raids into the region often failed and Ethiopian administrators and military personnel only resided in the towns of Harar and Jigjiga. Attempts at taxation in the region were called off following the massacre of 150 Ethiopian troops in January 1915.

During the 1920s and 1930s, there were no permanent Ethiopian settlements or administration in the Ogaden or any Somali inhabited land, only military encampments. In the years leading up to the Second Italo-Ethiopian War in 1935, the Ethiopian hold on the Ogaden remained tenuous. Due to native hostility, the region had nearly no Ethiopian presence until the Anglo-Ethiopian boundary commission in 1934 and the Wal Wal incident in 1935. Only during 1934, as the Anglo-Ethiopian boundary commission attempted to demarcate the border, did Somalis who had been transferred to the Ethiopian Empire by the British during the 1897 Anglo-Ethiopian Treaty realize that they had been placed under Abyssinian rule. In decades following the treaty, Somalis remained unaware about the transfer of their region due to the lack of 'any semblance' of effective administration of control being present over the Somalis to indicate that they were being annexed by the Ethiopian Empire.

Following the end of the Second Italo-Ethiopian War in 1937, and the outbreak of World War II, the Ogaden was united under a single administration with the British and Italian Somaliland's. After the defeat of Italy, power transferred to the British military administration. The British Foreign Secretary proposed to keep the Somalis territories unified after the war, but was rejected by the Ethiopians and France (then controlling French Somaliland) who wanted a return to the pre-war status quo. On 31 January 1942, Ethiopia and the United Kingdom signed the second "Anglo-Ethiopian Agreement", ending British military occupation in most of Ethiopia except Ogaden.