ISIS is not the massive organization that some politicians would like you to believe it is. They only have a few thousand adherents out of the more than a billion muslims worldwide. Just to hang on to the territory they currently control, they depend on their recruiters supplying them with a continuing supply of new fighters -- and they know that to actually win what they want, they need a massive amount of muslims to come over to their side. And to do that, they need to convince muslims that they are hated by the Western nations. Sadly, Donald Trump's obvious bigotry as he runs for president of the most powerful of the Western nations, is giving ISIS recruiters exactly what they need.
Here is how Justin Salhani describes it in Think Progress: