Sci Fi idea

Faster-than-light travel, wormholes, transporters, putting people in stasis so a trip only feels like a few days or hours: all that stuff doesn't work to get people moving long distances across space. The fastest we can actually get people going with rockets and ship-bound propulsion is not very fast, only a small fraction of light speed. Instead, the only way to get ships going fast (that is, close to light speed, which at an interplanetary scale is still pretty slow) is to create massive rings around stars that use photosynthetic energy to rapidly accelerate a ship, then release it to wherever it is supposed to go; gravity will be the brake at wherever it is going. Because people are going to be on these things for years, they've got to be big, which means the accelerators have to be big and thus difficult and time-consuming to build.

It takes months or years to get from one habitable planet to one of these star-bound accelerators, and then years from one solar system to another. So communication is slow, and actual contact is even slower. Wars revolve around controlling these accelerators, because that's what lets you get to other solar systems, but it takes months to make any kind of move even on these relatively nearby targets. So wars are long, Hundred Years War long, and people are in the midst of wars whose origins are older than anyone alive, which nobody can really remember.