Islamic Revivalist movements in Africa

So today in my History of Islam class we were discussing the different definitions of Jihad, because when most people here Jihad today in the news they associate it with terrorist attacks and Muslim holy wars. I learned that lesser Jihad, the militant struggle, should be avoided if at all possible and only used as a defense mechanism. We then proceeded to talk about lesser Jihad in 18th and 19th century Africa. Most of these “Jihads” were characterized by the notion of Islamic reform. The only legitimate Jihad came as way to stop corrupt leaders from selling Muslims into slavery. This begs the question what situations can be considered Islamic revivalist movements, and which constitute legitimate Jihad? Any thoughts?