Ahmadu Tall was a Toucouleur imam of Ségou (now Mali) in the nineteenth century. Ahmadu's father, El Hadj Umar Tall, conquered Ségou (then the heart of the Bambara Empire) on March 10, 1861. Not long afterwards, he began his conquest of the Fula empire of Massina, leaving Ahmadu as the imam of Ségou. After Umar Tall's 1864 death, his nephew Tidiani Tall succeeded him as head of the Toucouleur Empire. Wikipedia
The definitive Wikipedia entry for Ahmadu Tall. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Ahmadu_Tall