In movies and TV shows, some supposed enemy appears: an alien from another planet, some unidentified earth-based target, or something else that it appears the only way to defend against it (whether or not it has hostile intentions) is the Army, Navy, and/or Marines. So the military comes out, and they start shooting, and they discover the enemy has shields, or that for some reason (or lack of reason) the military's best ordnance attacks do not have even the slightest effect. Or, worse, their opponent has ray guns or other equipment that can shoot down and/or incinerate attacking equipment and/or aircraft.