Turkey had to get active and attack isis sooner or later, because the region controlled by isis between the pyd/ypg (part of pkk) parts would soon be 'liberated' from isis by pyd/ypg and then added to the pyd/ypg with the support of the US. Even if Turkey stayed out of Syria mess from the start, this day was bound to happen. Basically this operation is aimed at eliminating isis and bring fsa to this region, just so pyd/ypg can't conquer there first. Turkey will not allow pyd/ypg (part of pkk) to grab more land and later pose a bigger threat to Turkey. We shall see, Turkey has warned the pyd/ypg to leave Menbic and return to the east of Euphrates. If it doesn't and decides to clash with us, then pyd/ypg will be struck too.