Let me try to be more specific. Perhaps that can elicit a respons 
I was at war with the Torians, grinding them down surely but slowly. I was also allied with the Thalans and Altarians, so I would get a diplomatic victory by eliminatint the Torians.
Tha Altarians were allied with the Torians (and me) and was in the process of eliminating the Thalans.
I looked repeatedly for an option in the Diplomacy screen to get the Torians to surrender to me. It didn't appear. I wonder what it takes to get that option - it is not very apparent. Eventually, they surrendered to the Altarians, and I won the game a turn or two later. Before that, the Thalans also decided to surrender to the Altarians. I was an ally with the Thalans, so they could have decided to surender to me. I wonder why they decided to surrender to their enemy, contrary to the Torians, and not to their ally, as the Torians did.
As I said, I don't understand this diplomatic model, and I don't understand what to do to get the effect I want. I'll be happy to gain some insights from people who are more experienced with the game than me.