Stealth field generators allow individuals to effectively become invisible and have been in common use for centuries, most often seen in modern times in the possession of professional spies and underworld figures. Invisibility is also possible on a larger scale; occasionally private starships employ cloaking devices, but the costs involved mean they are reserved for the very rich, or those willing to seize such devices by force.
Despite the obvious advantages, in wartime cloaked ships and individuals are usually only deployed for small, covert operations. Cloaking an entire division or fleet would not be practical. But periodically, researchers or military leaders obsessed with the possibilities attempt to develop technology to allow mass-cloaking, knowing it would change the face of warfare forever. Given the amplification properties discovered in Ilum crystals, perhaps the time for change has finally come.