ISIS has had de facto control of most of what our maps say are Western Syria and Northern Iraq for about six months now.
The military efforts of the respective recognized governments of those states to regain control of their territory have been impotent. The military forces of the Kurdish regional government's milita have held back its advance, but he main Iraqi military has been nearly useless as have the civil war torn Syrian government's efforts to reclaim their territory.
Even if ISIS can be defeated, it is far from obvious that Syria or Iraq can effectively assert legitimate and accepted authority in these areas or the area controlled by the Kurdish regional government. Recent reports of indiscriminate killing of civilians and ISIS forces alike by Shi'ite militias entering ISIS controlled Sunni majority areas support a skeptical view.
There are examples of countries losing de facto control of their territory and regaining it after long periods of dispossession, like Columbia. But, they are rare.
Similarly, the Crimean annexation by Russia appears to be a fait accompli.