Originally, several groups of people lived in the area that would become the heart of the Songhai empire (known as the Gao region). For example, the Gao people hunted river animals along the Niger River, and the Do people farmed the rich mud in the river's floodplain. Eventually, these groups were conquered by another group of people, called the Songhai. The Gao civilization (which would become the Songhai empire) was born from these peoples.
As trading journeys across the Sahara desert became more common, Gao became an increasingly important trading center. Traders would make the journey from northern Africa across the Sahara to Gao, where they would come to meet traders from Ghana to the west. This established Gao as a small trading kingdom, and they began to expand, taking control of the area around the trade routes.
But their success would not go unnoticed. Around 1300, they were conquered by the then-dominant Mali empire, and were made to pay taxes to the Mali kings. This was the state of the empire in 1360, when Mansa Sulayman (the ruler of Mali at the time) died. There was a struggle over the succession, which weakened a Mali empire that was already unstable from earlier rulers who had spent money too liberally. The Songhai rebelled in Gao, and although the new leader of Mali tried to quash the rebellion, he didn't retake Gao. The Songhai were now independant once again. (The old kingdom was called Gao, because it was basically a city state. The newly independant Songhai now controlled more than one city, and so the name of the empire became Songhai, which was the name of the dominant ethnic group and language in the region.)
Sonni Ali's reign as king of Songhai began in 1462, and although he would only rule for thirty years, he would greatly expand the territory of the Songhai. He began to capture cities and expand the Songhai's influence. He used both his navy and army to conquer several important cities along the Niger River, such as Timbuktu and Jenne.
Askia the great's reign followed Sonni Ali's as king of Songhai. He conquered more lands, and added them to the empire. He was also more interested in learning than his predecessor, inviting scholars into Songhai. Songhai grew to its greatest size and power during Askia's reign, but when he was overthrown by his sons in 1528, the empire began to decline. In 1590, an army from north africa came with guns to conquer the Songhai empire, and the empire collapsed.