There are three things in life that's certain: Rollo being a traitor, Bjorn thinking with his crotch and Harald Finefair always scheming. Though I think you are giving Harald way to little credit and I believe his cleverness is actually his biggest weapon, combined with his ambition of course. He doesn't have the charisma or legendary name that Ragnar or Lagertha has and while being a good fighter he is not an outstanding fighter like Bjorn or Rollo are. His skill is that he knows exactly what to say to people to get the outcome he wants from them and is especially very good at playing into their ambition (probably because he can relate in that way). I would even say that (except for Eckbert) he is the best politician of them all and is extremly good at knowing when it's time to make an ambitious push and when it's time to fall back and not make a move. They talked about him conquering three different kingdoms during the 10 year time gap in season 4 (probably do to his wit more than his army) so obviously he is getting some results. That factored in with that his original kingdom apparently was very small and shitty compared to others I would say he is punching well over his weight class. Helps of course with having a very loyal and respected brother at his side as well. I also loved Eckberts lesson to Alfred and when he said "don't ever let yourself be influenced by other people, especially people like me", he is remarkably self aware of what a scumbag he is and is telling Alfred to watch out for other scumbags like him. In real life there were a lot of vikings that did settle in Normandy during Rollo's rule so his offer here was a nod to that. And Ivar declaring war on the whole world is a very viking thing to do, even if they did also trade a bunch with other people they still did fight and raid everyone and everwhere all over the known world.