After Virginia seceded from the Union on April 17, 1861, the only lines for overland supplies, troop movements, transportation, and communication to Washington, D.C., ran through Maryland, with the railroads running through Baltimore. Baltimore was a rough city for the Union, and Maryland an uncertain ally. In February, Baltimore rowdies had forced President-elect Lincoln to sneak through the city in disguise, and a mob attacked the Sixth Massachusetts Regiment as it marched through Baltimore on its way to Washington. Confederate sympathizers in Maryland were numerous, organized, and sometimes violent. The Maryland legislature was of questionable loyalty, prompting Lincoln to monitor its April 26 session and, later, to order the arrest of a number of its members.
Determined to keep the Maryland lines open, on April 27 Lincoln issued an order to General Winfield Scott authorizing him to suspend the writ of habeas corpus, at or near any military line between Philadelphia and Washington if the public safety required it. Lincoln issued his order pursuant to the provision in Article I, Section 9 of the Constitution stating that “the privilege of the writ of habeas corpus shall not be suspended, unless when in cases of rebellion and invasion the public safety may require it,” generally called the suspension clause.
On May 25, federal troops arrested John Merryman in Cockeysville, Maryland, for recruiting, training, and leading a drill company for Confederate service. Merryman’s lawyer promptly petitioned Chief Justice Roger Brooke Taney, sitting as a trial judge, for a writ of habeas corpus. This writ, sometimes called the Great Writ, is a judicial writ addressed to a jailer ordering him to come to court with his prisoner and explain why the prisoner is being held.