Kabuto didn't start the war, Obito did and Kabuto joined him only to get Sasuke from him. Orochimaru made Kabuto the way he is, which Orochimaru became further down the darkness from Danzo being his benefactor of all his experiments when he was in Konoha and Hiruzen fail to killing Orochimaru so it goes back to Danzo and Hiruzen more than to Tobirama. How was Madara able to awaken the rinnegan thanks to Tobirama? Was Tobirama who changed the Tablet info or was that Black Zetsu? Madara became how he did after reading the tablet and he tells the story to Hashirama during the war.
Danzo indirectly and directly caused the course to the war by affecting Sasuke, Uchiha memebers that were killed, Shisui, Itachi, Kabuto, Nagato, Konan, Yahiko, and Orochimaru directly. Indirectly he affected Obito, Naruto, Bee, A, and everyone in the war. I can tell you hold he caused each person I mention lives to change in a way to cause the war is you like.
It's sad when Danzo is more to blame as I said above^^^. He made the distrust against the Uchihas much bigger which made trying to find peace with them much harder for Hiruzen. He caused Shishi to die when Shisui only wanted peace for the village, he caused Itachi to kill his clan which caused Itachi to meet and team up with Obito to kill the Uchihas, join Akatsuki, manipulate Sasuke into the darkness which lead Sasuke to Orochimaru who is alive since Danzo and Hiruzen didn't kill him when they found out his darkness. Kabumaru became a byproduct of Orochimaru living which caused him to find Madara's body and join the war.