It lies with the magic(the reason)
Due to the brother choosing his sister over the country, the barrier weakened and the barbarians from ml's empire attacked them
The FL is like the main source of magic(the kernel) whereas her brother was more like the skull/source/body which can utilize the immense magic stored inside her sister
Also the FL and the ML have a supposedly old reaction of dragon(ML) and his disciple(FL)
The dragon gave his disciple a curse that he will never be able to forget his name and hence most of FL's magic is activated using the dragon's name haha
I see thanks for explaining, so they're attacked just for the sake of it, no grudge whatsoever, huh. I was kinda hoping they had a good reason lol.
They feared them cause the FL is a very powerful being who can tame the dragon.
But tbh they don't know the whole reason(?) They just wanted to take over the country like any other empire did back in the day.
Ugel(?) The barbarians were sent to conquer the kingdom and hence Argan was defeated cause the brother wanted to help the FL after she almost committed suicide once after not being able to handle her health/body.
But the reason totally lies with the magic.
I don't remember there being a mention about grudges or anything, so I think it's safe to assume they just wanted to conquer Argan.
Everything hasn't been completely explained from the ML's and the brother's side as well so we just gotta wait it out :)
I think I missed it, why were they trying to off the whole Argan, again?