On one hand, the republic would not have fallen if there had never been a sequel to Alien. Entire books could be written about how great—bordering very closely on perfect—Alien was as both science fiction and horror, not to mention how brilliant it was as cinema. Its unanswered questions are actually assets, deepening the mystery and thus the horror. But, on the other hand, those unanswered questions provided the basis for Aliens, a massively entertaining and actually quite moving piece of work.
What makes Aliens not only a sequel but a companion piece to Alien is the way it does for the action genre what its predecessor did for horror. Each picture is a master class in its respective genre, from narrative to design to acting, with every element integrated perfectly and contributing to making Alien a near-perfect SF horror movie, and Aliens a near-perfect SF action movie.