When you are being attacked by an empire currently you don't have any option to ask another empire to help you.

You could ask another empire to declare war but that is often not possible as the empire won't do that.


There should be an option to ask for assistance in diplomacy.


That assistance could range from sending fleets to defend your planets, giving you ships or as mentioned before all out war declaration.


For example Lumeris defending Sophons in exchange for tech's or just doing it for free to fend off Cravers that would come after them after destroying Sophons.