During a recent simulation, a siren erupted inside a bunker at Fort Greely, roughly 100 miles southeast of Fairbanks. Systems, sensors and interceptors were checked. Radars tracked a hypothetical, computer-generated incoming ballistic missile as it streaked high above the Pacific Ocean.

Before the mock missile could start the second half of its ominous flight path toward Alaska, a computer simulated the launch of an interceptor missile from an underground silo at Fort Greely, blowing the imaginary missile out of its virtual sky.