edited excerpts from:wikipedia
A writ of habeas corpus is a judicial mandate to a prison official ordering that an inmate be brought to the court so it can be determined whether or not that person is imprisoned lawfully and whether or not he should be released from custody. The writ is "the fundamental instrument for safeguarding individual freedom against arbitrary and lawless state action."
On April 27, 1861, habeas corpus was suspended by President Lincoln in Maryland and parts of midwestern states, including southern Indiana during the American Civil War. Lincoln did so in response to riots, local militia actions, and the threat that the border slave state of Maryland would secede from the Union, leaving the nation's capital, Washington, D.C., surrounded by hostile territory. Lincoln was also motivated by requests by generals to set up military courts to rein in "Copperheads" or Peace Democrats, and those in the Union who supported the Confederate cause. His action was challenged in court and overturned by the U.S. Circuit Court in Maryland (led by Supreme Court Chief Justice Roger B. Taney) in Ex Parte Merryman, 17 F. Cas. 144 (C.C.D. Md. 1861). Lincoln ignored Taney's order. In the Confederacy, Jefferson Davis also suspended habeas corpus and imposed martial law. This was in part to maintain order and spur industrial growth in the South to compensate for the economic loss inflicted by its secession.
In 1864, Lambdin P. Milligan and four others were accused of planning to steal Union weapons and invade Union prisoner-of-war camps and were sentenced to hang by a military court. However, their execution was not set until May 1865, so they were able to argue the case after the Civil War. In Ex Parte Milligan 71 U.S. 2 (1866), the Supreme Court of the United States decided that the suspension of the writ did not empower the President to try and convict citizens before military tribunals. The trial of civilians by military tribunals is allowed only if civilian courts are closed. This was one of the key Supreme Court Cases of the American Civil War that dealt with wartime civil liberties and martial law.
In the early 1870s, President Grant suspended habeas corpus in nine counties in South Carolina, as part of federal civil rights action against the Ku Klux Klan under the 1870 Force Act and 1871 Ku Klux Klan Act.
