The formation of Isis and its rapid expansion was a complete shock to the western powers. With the total chaos in Syria and Iraq, western intelligence services tried to map the countless number of militias involved in the civil war in Syria. It took some time for them to admit they have no clue as for potential allies to side with and support.
Since all potential allies may turn into a potential future risk, the west led by the U.S. spotted the only possible ally that could be trusted and is not involved in the attempt to over through the Syrian regime – the Kurds.
The Kurdish agenda has nothing to do with the Syrian or the Iraqi regimes. Their ambition is to have their independent entity. Colliding with Isis was a necessity as long as the fighting takes place in their potential home land. The Kurds have no will or capability to pursue Isis forces away from Kurdish territory. There's no doubt that the western powers came to some understanding with the Kurds. The "Pashmerga", the Kurdish armed militia, will carry out the fighting on the ground while the members of the coalition will provide air support and logistical backing. In return to the Kurds efforts, the western powers had promised the Kurds an independent state.The Kurds are keeping their part of the deal by paying with blood to secure what they see as their future state. They have no other choice. For the Kurds it is a matter of survival.
Will the western parties keep their part of the deal to grant the Kurds their independent entity?
From a strict military view point Isis is no match to a regular army. They, at the moment can't succeed in a total war with standing armies. Sooner or later, with a deepest involvement of the western countries, with boot on the ground, Isis will be defeated. Then, with the reorganization of the region to a new local political order, the Kurds will realize that they will be betrayed by the west.
The Kurdish territory borders with Turkey on the west and Iran on the East. An existence of a Kurdish state in that location is endangering the stability in Turkey and Iran. Both countries have a large Kurdish population that will no doubt aspire to join a Kurdish independent state.The Kurds will pay the price for the western ambition to fight Isis; however, the same west will not keep its part of the deal to give the Kurds a state.