The Abyssinian-Adal war was between the Ethiopian Empire and the Adal Sultanate that lasted for 14 years (1529-1543).
During this time, a Somali military leader named Imam Ahmad ibn Ibrahim al-Ghazi defeated several Ethiopian emperors and embarked on a conquest referred to as the Futuh Al-Habash ("Conquest of Abyssinia"), which brought three-quarters of Christian Abyssinia under the power of the Muslim Sultanate of Adal. With an army mainly composed of Somalis, Al-Ghazi's forces and their Ottoman allies came close to extinguishing the ancient Ethiopian kingdom. However, the Abyssinians managed to secure the assistance of Cristovao da Gama's Portuguese troops and maintain their domain's autonomy. Both polities in the process exhausted their resources and manpower, which resulted in the contraction of both powers and changed regional dynamics for centuries to come. Many historians trace the origins of hostility between Somalia and Ethiopia to this war.
In 1529, Imam Ahmad's Adal troops defeated a larger Ethiopian contingent at the Battle of Shimbra Kure. The victory came at a heavy cost but it solidified the Somali forces' morale, providing proof that they could stand up to the sizable Ethiopian army.
The victories that gave the followers of Imam Ahmad the upper hand came in 1531. The first was at Antukyah, where cannon fire at the start of the battle panicked the Ethiopian soldiers. The second was on 28 October at Amba Sel, when troops under the Imam not only defeated but dispersed the Ethiopian army and captured items of the Imperial regalia. These victories allowed the Somalis to enter the Ethiopian highlands, where they began to sack and burn numerous churches, including Atronsa Maryam, where the remains of several Emperors had been interred. The country was looted by the Ahmad's forces, who destroyed several Christian monuments and oppressed the non-Muslim Amhara and Tigray.
Dawit II died in 1540 and his heir was captured by the forces of Imam Ahmad; the Empress was unable to react as she was besieged in the capital. In 1543, Ethiopian guerrillas were able to defeat the Somalis with the help of the Portuguese navy, which brought 400 musketeers led by Cristovao da Gama. Although da Gama was captured in the Battle of Wofla, and later killed, in 1543 Ahmad himself was killed and his army destroyed in the Battle of Wayna Daga, with the help of surviving Portuguese musketeers. The Ethiopian/Portuguese force consolidated their victory by ambushing and destroying a second force under one of the Imam's subordinates. This turned the war around. The surviving Somalis were forced to withdraw from Ethiopia, leaving both kingdoms severely weakened.
It has been plausibly argued that because the participants in this conflict weakened each other severely, this provided an opportunity for the Oromo people to migrate into the lands south of the Abay east to Harar and establishing new territories.