Why did Curley fight with Lennie? What happened?

Curley has a major case of "small guy complex". Curley also thinks that Lennie, a big man, won't defend himself. He goes after Lennie because Lennie smiles while at a joke made at Curley's expense. Lennie doesn't defend himself until George tells him to. Lennie then crushes Curley's hand.

Curley puches Lennie in the face and He gets hurt on the nose and eyes. Lennie doesn't know what to do during the fight, so then George yells get him so then Lennie gets Curley by twisting his hand and he crushes Curleys hand. it's all bloody. He cries and Slim tells it's all Lennie's fault taht this happened. He tells Carlson to get the wagon ready, and Slim gets a cloth and wrappes Curleys crushed hand.