Right now Madara went of to fight Killerbee and Naruto. leaving Kakashi and Gai for Obito
as the fight progresses Obito will start talking to Kakashi about Rin (or being forced by Kakashi to talk about her).
Kakashi will state that he had no idea what happened or that it was an accident or that Obito imagined things.
Later in the fight Obito starts losing to Kakashi and gai (maybe the army joins in as well to help)
and as Obito is losing/dieing Madara suddenly appears next to him and will tell him how he set everything up for his own sake = death of rin / brainwashing Obito to become Tobi / reviving Madara etc..
as Obito finds out Madara used him for his plans (by setting up Rin's death) he will either go totally Berserk on Madara OR just realizes he has done terrible things and joins up with Kakashi, Gai and the army to take out Madara ..
just my idea ! let me know how you think about it please? :D