"That a single clan like Vltra is able to achieve all this...", sorry but you forgeting the help you had from a certain pirate clan in getting CM's and ports switched. Don't be ungratfull.
And this Casus Belli letter is just ridiculous, why they declare war twice at same nation? That hostility fleet was coming with peace in their holds? Maybe some humanitary help?
It seems you don't remember when your "Eastern Alliance" attacked 7 british ports in the same day. In those times was sustainable? Maybe it was... your Alliance lost all 7 PB's.
The rest of the pirates must do what the US players did in the past, you must fight them. In the past he tried to enforce his tyranny over US nation, they didn't give up and fought him.
Things are getting better, now atleast you offer diplomacy, in the past was more like backstabbing. Maybe was because RAE ruled the nation on those times.