Somebody (sorry, can't remember who!) asked why people think the US will start WW3.
I think they will simply because they're the ones who will actually declare it. You can push and push the West (led by US), and they'll put up with a lot. But when the line is crossed they will attack with everything.
At the moment Putin is pushing, trying to provoke a reaction. (I think he's rather a megalomaniac though.) Similarly, islamic State are pushing. Just depends on who pushes too far, thereby elliciting the reaction.
To be absolutely honest, my gut feeling re:Islamic State is that sooner or later they'll attack Jordan, at which point the West & allies will come in and crush them. As attacking Jordan would be the line that has been crossed.
Basically, WW3 will start when the US wants it to.