Making other races break alliances

Okay in my last game I was allied with the Arceans who were in turn allied with the Torians. I had already wiped out the Yor, Drengin, and Altairans. I wanted to take out the Torians, who were hostile but not at war with me, and have an alliance victory. But anytime I declared war on the Torians, who only controlled 5 planets, the Arceans would declare war on me. This made no sense! I wish there waas a diplomacy option that allowed you to demand that another race break its alliance with someone else. I finally got my victory in this game by wiping out the Torians in 1 turn. But I think making demands on who your allies are allied with should be an option.