All we know about is Tobirama hurt Izuna so heavy that he have to donate eyes to his brother, he have no choice in my opinion. Due to the raise of Senju family, Uchiha family already lost their willing to fight. That’s why Madara have to against Senju family alone, and beated by Hashirama finally (of course even though he own the Eternal Mangekyo Sharingan, he cannot defeat Hashirama’s Ninjutsu).