This is a pretty simple suggestion. Just found myself in this situation:
- Empire A is attacked by empire B (a devouring swarm).
- B conquers many of A's systems but A survives.
- Years later, I swoop in from a wormhole and declare war on B.
- I start conquering the systems that B took from A.
- My relations with A tank because A now has claims on me.
- As I am currently at war, I have no way to peacefully give A the claimed systems.
- Additionally, as I needed to travel through A's borders to get to the wormhole, A is now actively hindering my war efforts.