The Arachnids are a classic bunch of movie aliens, as weird looking as they are formidable. Their exact motivations and thoughts, however, are never stated in "Starship Troopers." Instead, viewers are left with the propaganda given out by the human military, and a line about the bugs possibly being provoked by human aggression, as the Federal Service entered their space. It appears that the human leaders simply want to wipe the alien species from the galaxy, leaving humanity as the sole interstellar power.
If this is the case, though, why doesn't the military use powerful nuclear weapons to simply destroy the alien worlds, rather than sending invading forces? Capturing a brain bug is one of Military Intelligence's aims, but this wouldn't even be necessary if the bugs were wiped out completely. The scientists would have no reason to study the Arachnids if they were no longer a threat. The armed forces certainly seem to have the power to do this, as Jean Rasczak (Michael Ironside) mentions his squadron will land on Tango Urilla after it's been glassed.
Other possible explanations don't hold up either. One excuse may be that the military wants to colonize the Arachnid planets, which would make destroying them disadvantageous. But every bug-controlled world we see is a rocky and apparently barren place that doesn't look particularly valuable. If the military really wants to defeat the bugs, a more devastating attack would make much more sense.
