I was talking about direct military conflict. That, I do sincerly believe this, is very unlikely.
I doubt you'll see US soldiers shooting at Russian soldiers in the next 5 decades. And if this would happen, we all have bigger issues at hand than ISIS.
And like Teeth wrote, internal disagreements between the various religious and political sides allows the IS to have this kind of military success. Some Sunnits defending the town, having good success, not getting any material support from Shiits at power cuz "HURRDURR they aren't trustworthy cuz <insert silly 2 millenia old stereotype bullshit religious reason>" and suddenly the brave defenders are trying to stop armored Humvees and tank with 25 yrs old Kalashnikovs. No surprise they fall back and it sure ain't because they are cowards, Mr. US-Senator.
(I might have mixed up the Sunnit/Shiit-sides - I tend to forget who is who cuz they are all stupid imho.)
If the reasonable forces would all just get over themselves and their shitty religion, this conflict could have ended already months ago. Everyone combined down there have easily a shitload more soldiers and better equipment than the IS has and could just end the miserable lifes of those cunts in a few weeks time. No involvement from US/EU/Russia/China needed really. They just need to get their shit together.
/rant