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Hasirama is one of the most OP characters in the Naruto series. No one could match his strength, even Madara, and he had crazy regenerative abilities. Basically, he had no weaknesses. Narutopedia sums it up pretty well:

Hashirama was famed as the strongest shinobi of his era, reputed as a God of Shinobi ... His body was brimming with vitality, granting him considerable regenerative powers that could mend most injuries with no residual effects ... Hashirama was proficient in all five basic nature transformations, along with Yin and Yang Release ... [It goes on and on]

So how did someone like him die in the First Shinobi World War? He alone has the same power as an entire village, and so long as he is careful, no one can even scratch him. And if he gets hurt, he can just heal it and shrug it off. So how could he so carelessly die?

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