The US Government should issue a Letter of Marque and Reprisal against ISIS. Any U.S. Citizens wishing to hunt and kill ISIS should be granted authorization to travel to any country where ISIS is operating to engage and destroy them. These citizens are considered Privateer forces, not subject to U.S. Military ROE. They may, for the period of their privateer activities, purchase any military hardware they can afford, such as machine guns or anti tank weaponry. Which they may take overseas to fight ISIS.
Any spoils of war such as gold, ancient relics, weapons, equipment, vehicles, etc, which may be captured by these privateers becomes their war booty which they may take home and sell to recoup the costs of their efforts.
Without involving the entire country this is a way to defeat ISIS