I agree with cleanmachine's post above. This is about landgrab facilitated by stirring division and creating fear as much as actual physical force.
For those saying we should do more - like what? When western countries become involved ISIS use it as propaganda to say "look, the western infidels are killing your muslim brothers, come and join us to fight back!" When governments in the region call on western governments to help them, ISIS say "look at how your government is in league with those infidels who kill your muslim brothers - come and join us to remove them from power"
It's a lose, lose, lose. We can't do nothing, particularly if a government in the region asks for help. We do get involved, it drives up support for ISIS in countries in the region and in our own western countries.
So what can we do? Actually, so much of this is an information and propaganda war that I do think there should be clampdowns on the content that is available online. Because whether driven by religious belief, egotism, boredom or sheer bloody delusion an alarming number of people - even in our own country - are looking at videos of shocking acts of murder and brutality and thinking it's a good idea to get involved.
But we won't stop it all, so we need to have messages that will reach the same audiences. Their videos work because they are highly graphic. Graphic images and slogans are easy to understand whether you have a degree or whether you can barely read, so they reach the widest spread of people. In short, they are a good recruiting tool.
Arguably, we need to match their propaganda with graphic images and slogans that counter their arguments. It won't win the war, but it just might help.
Finally, FWIW, I think Bush and Blair are partly accountable for this (terror acts by militant islam predate their election, after all, so they can't be wholly to blame) and am baffled by the idea that their foreign policy advisers could not see the complete clusterfuck that would unfold. So either they were warned and they ignored it, or the policy wonks who didn't see the risk should hang their heads in shame at their own ineptitude.