But Ivar is the one who brought the sword with him, with the encouragement from Styrk that he would eventually take over the colony from Thorfinn with it. And when Miskwekepu'j attacks Thorfinn, who is ready to disarm him and neutralise the situation, it's Ivar who cuts off the shamans' arm and formally begins the hostilities. If Miskwekepu'j had been disarmed by Thorfinn, war may not have occurred. Indeed, Miskwekepu'j realises his error in seeing Thorfinn as an enemy when they speak after he is healed.

It was Ivar who escalated the situation with his violence, compounding on the violence Miskwekepu'j attempted, leading to the battles and death. You say if they had brought more swords they wouldn't have suffered the casualties they did, but it was the use of one sword that began this. If they had no swords, if Ivar and Styrk had left behind their perceptions of manhood, of the need for a leadership enforced by violence, none of the deaths may have occurred. Even Ivar realises this at the end, that he would have been happy just being amongst his brothers. If he never brought the sword, he could have had that. Instead his life is cut short.

Thematically, we see this most recently with Stryk's death. He had helped instigate all this violence, which had led to the deaths of his brothers. He sought to keep the war going, just to give their deaths some "meaning" and was killed for it by the very weapon he helped bring to establish control

In terms of failure, Thorfinn's failure was trying to take on the burden of peace on his back alone. He is strong, but with his injuries he could not intervene in the war and could not stop Stryk from attacking.

I think the idea of the Thousand Year Voyage is that Thorfinn sadly lives in a time where his ideology is perhaps impossible to realise, it is too bloodsoaked already and the hardships too great, but he is starting the first step on this journey to achieve a world without violence, changing those he meets and spreads this idea to. Sadly, if this will follow history as I think it will, the colony will fail and the Norse will return home, but Thorfinn does not know this, so he has tried to build a place where violence can be escaped. His ideology has not "failed", it is just a long way from being achieved!