Tobirama caused most of the problems. Tbh, what Hashirama was planning to do was him being 2nd Hokage if anything ever happened to Madara and would of kept having Uchiha/Senju go turn by turn as Hokage one after another. That alone would of allowed the clans to strike a bond with each other and actually trust each other and not feel as if they were being oppressed as in Part 1.
But what i want to know is, the two strongest clans, Senju & Uchiha, were the dominant clans in Konoha in it's founding days. We know that the Uchiha were massacred, but why aren't there any Senju's left now other than Tsunade?
Maybe that will be explained in these flashbacks, aswell?