The Civil War: "he most profound civil liberties issue during the Civil War involved Lincoln's suspensions of the writ of habeas corpus. What is the writ? Suppose you are arrested by police or military officers while you're walking down the street. You or your representative has the right to go to a court and seek a writ of habeas corpus. This writ gives the court the power to order the government to justify its action and it gives the court the authority to order your release if it finds that your detention is unlawful. The writ of habeas corpus is the bulwark of our constitutional system. Without it, the executive branch could unilaterally seize you and you would have no right to have an independent branch of government determine whether the seizure was constitutional. You would be completely at the mercy of the President."

(Via Lessig Blog.)