We're not meant to know about Ciri's father's motivations yet in the show.
Some people think he's the messiah, others think he's a massive threat - that's all we've got to go on.
I haven't read all of the books, but I've heard some say that they expect his reasons for going to war to be that his motivations are bad, basically (and gross if they go with the books), and that because Geralt is sort of legitimately her father by the child-surprise, he can't just ask for her back.
And he wasn't exactly made welcome by Calanthe either - plus there are suspicious circumstances around his supposed death with Pavetta.
All adds up to trying to do it by force rather than being welcomed.