The invading army consisted purely of robotic lifeforms. Although they were all droids it was discovered they were powered by living energy. All attempts to rescue captured soldier failed as boarding parties were either repelled or no prisoners could be found on the vessels. This led to the belief that the prisoners were being harvested for their life force energy to power the alien technology, despite no transfer devices having been found.
The invading cruisers had an intelligence of their own unlike the smaller mechanisms. The intelligence of the larger vessels was strong enough to influence and corrupt the minds of boarding parties and sometimes people that spent extended periods in close proximity to the vessels. Force users appeared to be especially susceptible to the mind control effects of the intelligent vessels even though force users are generally stronger willed than non-force users. Not only were minds corrupted but powers were enhanced making even a novice force user a force to be reckoned with.
The ability for the invaders to corrupt the enemy made fighting difficult. The Chiss and Mandalorians adjusted their tactics to minimize close combat with the larger vessels. The Admiralty saw no need for this and turned to mercenaries for boarding parties. Very few mercenaries avoided corruption and survived combat with corrupted comrades to tell the truth of what was happening. The majority of a boarding party was corrupted at the same time which lead survivors to come to two conclusions. The first conclusion was that the corruption process affected most lifeforms in the same way. The second conclusion was that the corruption took root before any visible indications and the intelligent vessels waited to act until success was assured. The second conclusion created a lot of doubt in survivors as they could never be sure if an ally was secretly corrupted or even if he or she was and didn’t know it.
The governments that knew of the corruption and the purely droid armies kept that information a secret. They feared the panic that would be caused if the populace knew the invaders could produce and inexhaustible army that could control the minds of its enemies and turn brother against brother. The few mercenaries that survived and attempted to tell the truth were written off as crazy as the story was too unbelievable to many people. Like the governments, the mercenaries that knew the truth worked in silence to protect themselves from future invasions.