Those presently engaged in the fight against islamist Terrorism either have insufficient forces or insufficient resolve to win militarily.
DonDiego sees no immediate candidate to take up the cudgel against the advance of the Islamists. If the Islamic State has the means, as some have speculated, it is possible that Baghdad will be under siege by Christmas or New Years Day; hopefully US Embassy personnel will escape safely.
Perhaps once the Islamic State has recaptured the territory held at the peak of the Islamic Empire by the Umayyad Caliphate, they will pause to consolidate their gains.

Although the Battle for North Africa will have been relatively easy, as the strong men, President Mubarak of Egypt and The Brotherly Leader and Guide of the Revolution of Libya Gaddafi, have already been removed by The Obama of the United States. It is likely they'll choose to to recuperate from the brief nuclear exchanges during the battle of Jerusalem anyway.
Boilerman's question, " . . . who will clean it up?", presupposes someone
will clean it up. This is not necessarily so; throughout mankind's history totalitarianism has held sway more frequently than democracy. But, if, . . . or more likely
when, . . . the Islamist Caliphate crosses paths with China or Russia, they will have met their match.
The inevitable decline of civilization - interrupted briefly by unprecedented technical and social growth displayed in the 18th, 19th, and 20th centuries - will resume.
The days of "Happy Motoring" will be over.