Non-Aggression Principle only works if everyone agrees to it. If you are the only guy doing it everyone else will stick you with the bill

Which is why one of the ways Game of Thrones ended was even more retarded when Sansa took The North out of the Seven Kingdoms. Before that all 7 were generally considered a united group so an attack on one was met with force by the others, more so when that attack came from one of the others like was the case in the book history when the pirate nation attacked various others. Theon being a ward of the Stark's was part of the fallout of that.

TV show ended with Sansa seceding from that agreement, which was hilariously stupid because originally the pirate nation was one of the groups that wanted to do it but then didn't because the writers probably forgot about the plotline, so now anyone that attacks The North can do so with impunity, even if it happens to be one of the other six remaining kingdoms they were allies with only until recently!