No power in history has ever defeated a "Guerrilla" force that rose up in opposition to those who sought to impose their will upon them [or as representatives of their countrymen, albeit that not ALL of those countrymen be dissidents] and the fact that "Peace" may be reported at the end of the conflict, it is always achieved at severe cost to the aggressor.
Of course, the usual "Cop-out" is for the imposer to declare itself victor as a "Face Saver" but invariably, the truth is that their failure is consigned to the pages of history and kept out of the reach of the those who did not participate in resistance.
Problem NOW is, the forces of IS have attained a level of strength and capability not anticipated by those forces of the West that imagined they would be fighting a handful of disgruntled Arabs who could be dispersed by forceful threats of military superiority, they have burned their fingers this time and there is much talk of "Negotiation" in high places.........[watch this space].
Ultimately, there WILL be a "Caliphate", it WILL be ruled by "Sharia Law" and it WILL be the result of the efforts of the forces of IS.
Better to get used to the idea, the writing is on the wall.