How did Islam get spread in Saharan and sub-Saharan Africa?

As Islam has always spread - by the sword. A lack of viable defenses led nation after nation to fall to Islam. ... And the tyranny of "convert, pay, or die" (a offer that the natives couldn't refuse) allowed for Islam to establish itself.

Ignorance mostly. Like how they can get a child that only tends sheep to strap a bomb around himself and go blow himself up thinking he will have glory in heaven.

There are plenty of books about this in your library. Go read one. Ignore all the one-sentence answers you're going to get here from idiots who don't know what they're talking about (especially ones that contain the word "sword" in them).