Banzoku Tensei! - Make Ikusa Kara Hajimaru Isekai Seifuku

Reborn into a low-tech barbarian tribe!? In this story of rising to power, the protagonist is reincarnated as the chieftain of a defeated barbarian tribe, subjugated by a powerful empire. With their tribe on the verge of collapse after losing a war, the protagonist leads them in a fight to overthrow the empire and reclaim their freedom.