Birth of the Empires

I am playing a game as the coalition of planets. While exploring, i came across a system controlled by a war like minor race. They declared war on me a couple turns later. I have several starships in their system bombarding it. The war is growing tiresome though, and I want to end it. I'm the diplomacy screen though, I don't see an option for cease fire, end the war, or anything like that. How do I end the war?

You should be able to do it just in this diplomacy screen. But I can't tell you exactly until I try it again myself, in a few days.

Either by accident, because the minor suddenly likes you because of a change of government.
Active through you because you give him gifts for so long. The duration depends on how easy or difficult it is to impress you.