What caused the mali empire to crumble and the songhai to rise up?

In the ninth century C.E. and in the ninth century, when the inhabitants of the city of Gao along the Niger River expanded to the Middle Niger region, the Songhai State emerged. Songhai flourished in river trade, focusing mainly on the exchange of agricultural produce, fishing, hunting and technology for ironwork. The power and prosperity of the Songhai state grew beyond their participation in the Trans-Saharan trade. Berries traders from North Africa and producers from the forests of West Africa exchanged gold, salt, ivory, skins, and copper, among other goods.

The prosperity of Songhai attracted the attention of the powerful Empire of Mali, which incorporated the state in the fourteenth century. As Mali weakened, the leaders of the Sonni dynasty regained Songhai's independence and began to expand their borders in the 15th century, leading the Songhai’s cavalry and canoe fleets to defeat neighbouring forces and conquer the prosperous cities of Timbuktu and Jenne in 1473.

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four sitting presidents have been assassinated while in office but the first was abraham lincoln,