The "French Soudan" or "Western Sudan" should not be confused with the modern country of Sudan, or with the colony that was known as "Anglo-Egyptian Sudan" during most of the 20th century. Although similar in climate and latitude, the British Sudan was located on the east side of Africa near the Nile River while the French Soudan was located west of Lake Chad. The French viewed the Soudan as the link between their holdings in Algeria and Senegal, as well as the gateway to the Congo via Lake Chad.
Consequently, the French exhausted an enormous amount of effort to claim it even though its economic value was small.
Prior to the 1850s, there was no French presence in the Soudan, although French explorer René Caillié passed through the area enroute to becoming the first European to return from Timbuktu in 1827-1828. In 1855 Faidherbe's troops began their advance towards the Upper Senegal River and built a fort at Médine, just below the Félou waterfall. Beyond that lay the empire of Ahmadu Tall with its capital at Ségou on the Niger River, and the empire of Samory Touré to the south east, with a capital at Sanankoro on the south side of the Niger River.
For a time, the French felt no need to formalize their presence, but as the British began to move inland from the mouth of the Niger River, the French began to fear that the British would seize the Middle Niger Valley and perhaps even reach the Upper Senegal Valley. That fear inspired a new episode of French aggression from 1876-1881 when Governor Brière de l'Isle authorized the construction of a railroad to the Niger and the creation of a military government of Haut Senegal et Niger (literally "Upper Senegal and Niger"), an area encompassing the modern states of Mali and Niger.
The African empires offered resistance, so it took a decade for the French to conquer Ségou and almost twenty more years to defeat Samory. Resistance continued in the region around Lake Chad until 1900 and in the Sahara Desert until the 1930s. But by 1899, the French were able to place the Soudan under its own administration.