What happens when one spouse sues the other on fault grounds?

In this case, the plaintiff, who is the party initiating the action, claims that the defendant is at fault for the failure of the marriage. Grounds are the reasons for a divorce, and fault is saying, essentially, that it’s someone’s fault that these grounds exist. Adultery is a classic fault ground.

Fault means that one spouse has done something wrong. Adultery is one of the most common and oldest fault grounds because marital infidelity was held to be worthy of harsh social condemnation.

Other fault grounds, which vary from state to state, include cruelty, both mental and physical; abandonment or desertion as well as deviant sexual conduct, extreme cruelty (also called cruel and in human treatment), habitual drunkenness, mental illness, sexual desertion, drug addiction, and nonsupport.

Cruelty doesn’t mean just being mean to one another, but rather that unnecessary physical or emotional pain is being inflicted upon one another. Abandonment means that one spouse has left the other without the other’s consent. The length of time of the abandonment varies from state to state.
Under traditional fault grounds, one spouse could defeat the claims of fault by the other spouse by raising certain defenses. Under a no-fault system, one could still litigate or argue in court the divorce claims, but the traditional defenses do not apply.