@Arkos
Your original point, as I understood it, is that if you want peace, be prepared for war, and again, I'm saying that's an untrue statement. Both sides having weapons and being prepared for war does not guarantee peace. At all. The only thing that'll be guaranteed is that they'll be used. Long lasting peace comes from diplomacy, shared ideals, shared values, and trade. Literally talking. That is Thorfinn's plan. Has it worked in real life? Yes. Has it failed in real life? Also yes. Will it work in the story, I mean... probably.
Your other point, that intent is what matters, not the actual bringing of the sword, is not really true either. Bringing the sword shows Eynar's intent: to use a sword. He shows that his intent is not peace. Yes, he can use a spear to kill people, and yes, spears are probably more effective than swords, but I think you're being too literal with that Thorfinn is saying. It's about what the sword represents. The sword is a tool that is only meant for killing people. If someone had said they want to bring a war axe, or mace, or a flail, or whatever, the answer would be the same. This is a "read between the lines" moment. If you can't give up your sword, the killing tool you seem to be very comfortable with, then you don't want the same thing that I do. Leave it behind or don't come. Your weapon, and it's intended purpose, is going to lead you to a place where you're going to use it for its intended use, and I don't want that in Vinland.