If I want to explain in very very simple words I would put it this way:
If I wanted to destroy a muslim country, and I didn't have enough good reasons, the whole world would become against me.
What I would do is I make up these little terrorist groups to represent the world of Islam. And since I own the media I'd be showing my own creations 24/7. Naturally, there are people who fall to join these groups without knowing the truth behind it. And but by bit it turns into a real civil war.
And I gain the privilege to increase my influence in that region (war on terror huh?)
Afghanistan, Pakistan, Iraq, Syria are examples. Next up is Iran. ISIS is originally made to represent Iran. You have probably heard of Khorasan Group which is like a second name for ISIS and there's only one place called Khorasan and that's in Iran. I can write a book about it but.. if you want to find the big "WHY" you just have to search for these words:
Twelfth imam - khorasan - black flags (however, you only find the truth in true islamic sources.)