They didn't.If that were true, they wouldn't of killed each other off.
Edit:
so basically, i'm supposed to just take your word for it despite not being able to produce a single documented record of Ashura reaching out to Indra, considering neither Black Zetsu nor Hagoromo gave the slightest thing that even remotely suggested it. It's a simple enough request to produce something that says it, and seeing as though you can do everything but that, it further tells me you got nothing.
It takes 2 parties for there to be a war, and if Ashura was really that loving, that it stands to reason there would be some account of him extending an olive branch. And there isn't a single thing that says it since Black Zetsu's accounts said Ashura was apparently more concerned about defeating Indra than working with him.
Let me just ask you this, do you believe Hashirama was responsible for the wars with Madara?