Why did Shanks stop the war?

I think Shanks stopped the war mostly because of the senseless killing. He had the same thought as Coby, the marines main priority was accomplished and he simply just stepped in and reminded the marines that.

Who defeated Shanks?

He also fought against two Yonko, Shanks and Kaido, but was utterly defeated by them. Shanks cut off his arm and Kaido beat him to a pulp, which makes it clear that as of now, Kid won’t be able to defeat Shanks.