Complex sentence: Unfortunately, without any means of communication between the two species, humanity did not realize this fact and, fearing another Formic invasion, made the decision to assault the Formics' homeworld and wipe out the alien threat, once and for all.
Split into simpler sentences: Unfortunately, without any means of communication between the two species, humanity did not realize this fact. Fearing another Formic invasion, it made the decision to assault the Formics' homeworld and wipe out the alien threat, once and for all.