Board Thread:Theories and Speculation/@comment-26904721-20150820181613/@comment-5936871-20150821172400

Considering both Hashirama and Tobirama died in the First Shinobi World War. I think it's most likely that since the Senju Clan was the main founding clan of Konoha, and Hashirama was it's Hokage and Head of the Senju Clan. With Tobirama taking over those positions after Hashirama died. The other Senju most likely died during the war as well against the other nations. I mean, they may have been the strongest clan in the world. But that doesn't make them invincible, and if they're severely outnumbered then yeah they would definitely all die. I mean they were basically it's creators alongside the Uchiha, so they'd probably put themselves on the frontlines alongside Hashirama. Knowing his personality, I can see him putting himself right into the heart of battle against the other nations. However, Hashirama definitely shouldn't have died, he was basically a God compared to anyone else. The only explanation I can come up with for Hashirama's death is that all four of the other first Kage ganged up on him and managed to killed him in an epic battle, with them all dying in the process as well. With their positions being taken by the second Kages, which would explain why all of the second Kages knew each other and seemed to be within the same generation. Currently, now Tsunade is the only known survivor of her clan.