Plot hole: The facilities at Ft. Hood have working electricity to power the simulators, projectors, etc. even though it's been 1000 years, with no logical reason for the Psychlos to have kept the facility maintained, and the fact that the Psychlos should by rights have leveled the place when they invaded 1000 years before. Even automatic backup generators would have no fuel after 1000 years dormancy except for a nuclear system, which would still have required regular maintenance over a 1000 year interval to maintain automatic functionality.
Plot hole: After 1000 years have passed (without any maintenance) the Harrier jets still function like new. Amazing.
Plot hole: The barbarians find a working 1000 year old flight simulator. Beside the fact that the machine should not be working to begin with after such time, they were lucky enough to find a working power source in the barren wasteland.
Plot hole: Terl says that the army of humanity (presumably the U.S. Army) could only put up a nine minute fight against the Psychlos, and yet in the end of the film, a couple of outsiders, who taught themselves everything they know in just two weeks, can defeat their army. Why? The outsiders are outnumbered, they have had zero combat-experience, they use weapons that are over a thousand years old. That's just amazing.
Chosen answer: It is residual radiation from atomic weapons used in an attempt to fight off the Psychlos.