President Lincoln suspended the writ of habeus corpus. Habeus corpus requires that a person under arrest be brought before a judge or into court, and unless sufficient legal grounds can be shown for their detention, then they must be released.
This move started when a state legislator from Maryland, John Merryman, tried to stop Union troops from moving from Baltimore to Washington. He was arrested and held at Fort McHenry by Union military officials. Merryman's lawyer requested a writ of habeas corpus to have a federal court review the charges. That's when Lincoln first suspended the right of habeas corpus. The military officials at Fort McHenry kept Merryman in custody. Other instances would follow. Pres. Lincoln was firm in his insistence that he needed to suspend certain rules in order to put down the rebellion in the South.