Habeas corpus is another Latin phrase, meaning "(you should) have the person," and it's part of a longer phrase, habeas corpus ad subjiciendum, meaning "(you should) produce or have the person to be subjected to (examination)." These were the opening words of writs in 14th century English legal documents to require a person to be brought before a court or judge, especially to determine if that person is being legally detained.
Basically, habeas corpus represents the legal right that a person in a free society has to not be whisked from his or her home without reason or cause and to not be detained or punished by the authorities without getting a fair hearing in court and a chance of self-defense. William Rawle in 1829 called the writ, "the great remedy of the citizen or subject against arbitrary or illegal imprisonment; it is the mode by which the judicial power speedily and effectually protects the personal liberty of every individual, and repels the injustice of unconstitutional laws or despotic governors."
Article 1, section 9 of the Constitution, restricting powers of Congress, forbids the suspension of habeas corpus except, "when in Cases of Rebellion or Invasion the public safety may require it." On April 27, 1861, about a week after the Fort Sumter surrender, President Lincoln ordered Winfield Scott, then head of the nation's military, to arrest anyone between Washington and Philadelphia suspected of subversive acts or speech, and his order specifically authorized suspension of the writ of habeas corpus. Scott passed the order down the line, and Southern sympathizers in Maryland were rounded up in batches.
This was during the crucial first weeks of the war, when Washington, D.C., desperately needed troops to defend itself and the northern regiments were having difficulty crossing Maryland, which had secessionist sentiments and was hostile to the idea of being overrun by the federal army. The Maryland legislature was about to meet, and...