A writ of habeas corpus is used to bring a prisoner or other detainee (e.g. institutionalized mental patient) before the court to determine if the person's imprisonment or detention is lawful. A habeas petition proceeds as a civil action against the State agent (usually a warden) who holds the defendant in custody.
The union has more states. Since, the union has more states that means they have more advantage because they have more power than the confederate states.