Line in the Stars
Two centuries ago we discovered alien life, or it discovered us. Ten years ago, we discovered why we'd never met it before. Humans always thought they were alone in the universe, that life was uncommon enough it had just missed us by hundreds of millions of years. Even the gate drone that entered our system, carrying with it data on a few over five hundred star systems and links to the gateways orbiting them
After two centuries of work on the gates and an easing of restrictions on AI, cybernetic and genetic engineering (after the most radical and destructive elements of those who embraced such technologies had left for the edge of the five hundred) we made a breakthrough: skip drives, allowing us to finally move off the net and explore those star systems which seemed to have no jump probes.
What we didn't know, was that there are laws against such things. The entities that had created the gate drone didn't take too kindly to us leaving the route they'd laid down for us, carefully keeping us away from fallow worlds, species and secrets they considered us unready for. Action was taken to police us. The creators didn't bother to attack us themselves of course, it's unlikely they're even in a form that can interact with us really. Instead a coalition of local allies formed and moved into police us in their stead.
The fighting lasted five years. Humanity lost, but that defeat was not the end. Our enemies found destroying our will to fight too costly, and left the job half finished. Humanity can recover, and next time we cross the Line in the Stars, it can be different.