Breaching The Unbreachable

See There Can Be Only One, and be sure to max reputation first for Our Survival Is Assured

If, after maxxing reputation, you're unable to attack then select their homeworld and in advanced planet options (bottom-right corner) there's an action to break the truce with them (permanently!). Do this and after a 30-seconds-ish wait you'll be at war immediately.

Also, it has to be you that destroys them; if another faction does so you don't get the achievement. Hence this is soft-missable.