Though there are several candidates for the strongest ninja in the world of Naruto, it's undeniable that the First Hokage, Hashirama Senju, is a top contender. Despite his strength, however, he died at a young age, with no explanation given in the series. It's likely that he died in the First Shinobi World War, which leaves a question: who could have killed someone labeled «the Shinobi God?»
Hashirama was the heir of the Senju, one of the most powerful clans during the Warring States Period, when the constant conflict between countries forced even the children of the mercenary shinobi clans to go to war. At a young age, Hashirama met a member of the most powerful rival clan, Madara Uchiha, and the two started dreaming of a way to end the conflict. They founded Konohagakure, the first of the shinobi Hidden Villages, and unified the Senju, Uchiha, and their affiliated clans, starting a brief era of peaceful coexistence. Hashirama became the First Hokage, but Madara grew disillusioned with their dream and started attacking Konoha. The two were regarded as the most powerful shinobi in history, and yet Senju always bested Madara with ease, even when the latter subdued Kurama, the Nine-Tailed Fox.
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Senju's powerful abilities come from his heritage as a descendant of Asura Otsutsuki, the younger son of Haguromo, aka the Sage of the Six Paths, the god and ancestor of all shinobi and the son of the alien goddess Kaguya. The Senju inherited Asura's powerful life force and chakra, which Hashirama eventually manifested his unique Wood Release Jutsu. The Wood Release Jutsu allowed Hashirama to subdue and control the dangerous Tailed Beasts, which let him
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