Adal occupied the lowlands east of the Ethiopian highlands, a territory stretching from the ancient port of Zeila on the Gulf of Aden to the walled city of Harar on its inland plateau, and at its peak all the way to Cape Guardafui at the tip of the Somali coast. The name itself likely derives from the Somali word Awdal, and the region's modern Awdal province in Somaliland still carries it. The sultanate's wealth flowed through its ports: millet, cattle, ivory, gold, and slaves moved outward while arms, horses, and manufactured goods moved in. Its merchants used dinars and dirhems. Its cities, Harar, Berbera, Zeila, Dakkar, Amud, boasted courtyard houses, mosques, shrines, and walled enclosures that connected the Horn of Africa to the commercial networks of the Red Sea and Indian Ocean. The Walashma dynasty sat at the top of this system, though real power increasingly resided with the military emirs who administered the provinces.

The Adal Sultanate's greatest strength was also its deepest vulnerability: the relationship between its hereditary sultans and its warrior emirs. Sultan Jamal ad-Din II was perhaps the most capable ruler the dynasty produced. He reorganized shattered armies, defeated Solomonic forces at Bale, Yedeya, and Jazja, and chased Emperor Yeshaq to the Blue Nile. But Jamal was assassinated around 1432, likely by his own kinsmen. His brother Badlay collected funding from Muslim kingdoms as far away as Mogadishu for an ambitious offensive into the Ethiopian highlands, only to die in defeat at the Battle of Gomit. When the next sultan submitted to the Ethiopian emperor and began paying tribute, Emir Laday Usman of Harar marched to Dakkar and seized power in 1471, leaving the sultan as a figurehead. From that point forward, Adal was effectively ruled by emirs, most notably Mahfuz, who launched relentless raids into Christian territory despite the nominal sultans' preference for peace.

In 1529, Imam Ahmad ibn Ibrahim al-Ghazi, known to his enemies as Ahmad Gragn, the left-handed, launched what no Adalite ruler had achieved before: a systematic conquest of Abyssinia. After a decisive victory at the Battle of Shimbra Kure, he returned to Harar to reorganize his forces, struggling to control Somali nomads who dispersed with their plunder and Harari troops who feared relocating their power base. By 1531, he was ready for a permanent occupation. The campaign was a succession of conquests: Dawaro and Shewa fell in 1531, Bete Amhara and Lasta in 1533. By 1535, Ahmad's armies had reached the coast of Medri Bahri, and Emperor Dawit II had become a fugitive, hunted from Tigray to Begemder to Gojjam. The Adalites occupied territory from Zeila to Massawa. In newly conquered regions, many Abyssinians converted to Islam. But in the heartland provinces of Bete Amhara, Tigray, and Shewa, populations resisted bitterly, with some choosing death over conversion.

The conquest of Abyssinia drew in outside powers. Portuguese musketeers arrived to support the beleaguered Christian emperor, while the Ottoman Empire supplied Adal with firearms and soldiers. What had been a regional conflict became a proxy war between European and Ottoman interests in the Horn of Africa. Ahmad's death in battle eventually turned the tide, though the exact circumstances belong to a later chapter of Ethiopian history. After the conquest's collapse, Adal's internal fractures resurfaced with lethal speed. Sultan Abu Bakr ibn Muhammad moved the capital from Dakkar to Harar in 1520, but the sultanate was torn apart by factional struggles in which five sultans succeeded one another in just two years. A capable leader named Garad Abun Adashe restored order briefly before being ambushed and killed at Zeila in 1525 by the sultan's Somali recruits. By 1577, what remained of the sultanate moved its capital to Aussa, far from the glory days when Adalite armies had marched across the Ethiopian highlands. The walled city of Harar, where much of this history was made, still stands.