Board Thread:Theories and Speculation/@comment-1018594-20150111145319/@comment-8284722-20150111220446

Joking aside, we know that Hashirama was KIA in war, likely the First Shinobi World War. We also know Tobirama was by his side, and that he entrusted the Hokage position to him while also telling him to tolerate the Uchiha. Who/what exactly killed him is unknown, though. Tragically, Tobirama suffered a near-identical fate, with Hiruzen at his side.