The whole game we've been bros. We defeated nations together, had dozens of mutually beneficial trade routes, I gave him everything he ever asked for when his "nation was in trouble".

Then recently, I caught him spying on me. I forgave him, and I thought we were good. But when we had finally defeated all the world's enemies, those self destructive nations who purposely step on your toes and seige war on you for no reason, he decided to turn on me.

There was no reason. No "betrayal" on my part. It had to happen because the game decided there can be no peace. You have no friends in Civilization.