If the U.S. government decided to declare war on ISIS, it would only be a simulated war, so that they would looked good in front of the American people. Why? For the same reason that the U.S. government put ISIS in place in the first place... to have a job to do, protecting Americans from some imaginary enemy.
In other words, if the U.S. government wasn't intentionally creating enemies around the globe, America wouldn't have any enemies. Then there would be no need for the U.S. government. And all those government people would have to go out and seek employment in the private sector.
There would be nothing wrong with this, but it is so much easier collecting big government pay while not having to do any work, than it is working at real labor for a pittance.