Hotfix build 79600 is live!

"Annihilation is final"
You can get around it if you were to quit the game before you get annihilated.
Also on larger systems you get more tech before you have to fight
There are mods on PAMM, the mod manager, to allow you to see what tech/ai difficulty is on the next node, so that you can plan accordingly.
You can have multiple galactic wars running, and they are listed in the "active wars" tab.
You can also just play AI skirmishes if you really want. Galactic war is designed to be challenging.
Also try playing on the normal difficulty, where they do nothing.