Muhammad could have been stopped in Medina before he grew very powerful. Muslims were greatly outnumbered for a long time. But the residents of Medina had business to attend to and were content to let somebody else stick their neck out to deal with it, and besides, what was Muhammad and his weak little band going to do? Take over the whole town? As a matter of fact, they eventually did.
The more Muhammad asserted his willingness to murder, the more scared the Medinans were to speak up — which caused their poets (their equivalent of our modern day political cartoonists and Op-Ed writers) to silence themselves out of fear, which hindered the Medinans from expressing their mutual feelings of rising discomfort, which might have joined them together into a united resistance against their common enemy. So the Muslims were able to defeat and eliminate one tribe at a time until they ruled the town and imposed Islam on anyone left alive.
This is a deadly ideology, and it is "clever" in the same way that viruses are "clever." Deadly viruses are efficient and functional in a way that makes them hard to defeat.
We're going to have to be smart. We're going to have to muster our courage to act, and to prevent ourselves from giving up in despair. And we're going to have to unite as many of us in this cause as we can. The stakes can't get any higher.