During the American Civil War (1861–1865), Maryland, a slave state, was one of the southern border states, meaning that it shared a border with a non slave-holding state of the North. Despite some popular support for the cause of the Confederate States of America, Maryland did not secede during the Civil War. Governor Thomas H. Hicks, despite his early sympathies for the South, helped prevent the state from seceding.
Because the state bordered the District of Columbia and the opposing factions within the state strongly desired to sway public opinion towards their respective causes, Maryland played an important role in the war. The Presidency of Abraham Lincoln (1861–1865) suspended the constitutional right of habeas corpus from Washington to Philadelphia. Lincoln ignored the ruling of Chief Justice Roger B. Taney in "Ex parte Merryman" decision in 1861 concerning freeing John Merryman, a prominent Southern sympathizer arrested by the military.
The first fatalities of the war happened during the Baltimore riot of 1861 on April 18–19. The single bloodiest day of combat in American military history occurred during the first major Confederate invasion of the North in the Maryland Campaign, just north above the Potomac River near Sharpsburg in Washington County, at the Battle of Antietam on September 17, 1862. The battle of Antietam, though tactically a draw, was strategically enough of a Union victory to give Lincoln the opportunity to issue, in September 1862, the Emancipation Proclamation. It did not affect Maryland. In July 1864 the Battle of Monocacy was fought near Frederick, Maryland as part of the Valley Campaigns of 1864. Monocacy was a tactical victory for the Confederate States Army but a strategic defeat, as the one-day delay inflicted on the attacking Confederates cost rebel General Jubal Early his chance to capture the Union capital of Washington, D.C.
Across the state, some 50,000 citizens signed up for the military, with most joining the United States Army. Approximately a tenth as many enlisted were to "go South" and fight for the Confederacy. Abolition of slavery in Maryland came before the end of the war, with a new third constitution voted approval in 1864 by a small majority of Radical Republican Unionists then controlling the nominally Democratic state.
Maryland, as a slave-holding border state, was deeply divided over the antebellum arguments over states' rights and the future of slavery in the Union. Culturally, geographically and economically, Maryland was a unique blend of Southern agrarianism and Northern mercantilism. In the leadup to the American Civil War, it became clear that the state was bitterly divided in its sympathies. There was much less appetite for secession than elsewhere in the Southern States (South Carolina, Mississippi, Florida, Georgia, Alabama, Louisiana, Texas, Virginia, North Carolina, Arkansas, Tennessee) or in the border Southern states (Kentucky and Missouri) of the Upper South, but Maryland was equally unsympathetic towards the potentially abolitionist position of Republican candidate Abraham Lincoln. In the presidential election of 1860 Lincoln won just 2,294 votes out of a total of 92,421, only 2.5% of the votes cast, coming in at a distant fourth place with Southern Democrat (and later Confederate general) John C. Breckinridge winning the state. In seven counties, Lincoln received not a single vote.
The areas of Southern and Eastern Shore Maryland, especially those on the Chesapeake Bay (which neighbored Virginia), which had prospered on the tobacco trade and slave labor, were generally sympathetic to the South, while the central and western areas of the state, especially Marylanders of German origin, had stronger economic ties to the North and thus were pro-Union. Not all blacks in Maryland were slaves. The 1860 Federal Census showed there were nearly as many free blacks (83,942) as slaves (87,189) in Maryland, although the latter were much more dominant in southern counties.
However, across the state, sympathies were mixed. Many Marylanders were simply pragmatic, recognizing that the state's long border with the Union state of Pennsylvania would be almost impossible to defend in the event of war. Maryland businessmen feared the likely loss of trade that would be caused by war and the strong possibility of a blockade of Baltimore's port by the Union Navy. Other residents, and a majority of the legislature, wished to remain in the Union, but did not want to be involved in a war against their southern neighbors, and sought to prevent a military response by Lincoln to the South's secession.
After John Brown's raid on Harpers Ferry in 1859, many citizens began forming local militias, determined to prevent a future slave uprising.
The first bloodshed of the Civil War occurred in Maryland. Anxious about the risk of secessionists capturing Washington, D.C., given that the capital was bordered by Virginia, and preparing for war with the South, the federal government requested armed volunteers to suppress "unlawful combinations" in the South. Soldiers from Pennsylvania and Massachusetts were transported by rail to Baltimore, where they had to disembark, march through the city, and board another train to continue their journey south to Washington.
As one Massachusetts regiment was transferred between stations on April 19, a mob of Marylanders sympathizing with the South, or objecting to the use of federal troops against the seceding states, attacked the train cars and blocked the route; some began throwing cobblestones and bricks at the troops, assaulting them with "shouts and stones". Panicked by the situation, several soldiers fired into the mob, whether "accidentally", "in a desultory manner", or "by the command of the officers" is unclear. Chaos ensued as a giant brawl began between fleeing soldiers, the violent mob, and the Baltimore police who tried to suppress the violence. Four soldiers and twelve civilians were killed in the riot.
The disorder inspired James Ryder Randall, a Marylander living in Louisiana, to write a poem which would be put to music and, in 1939, become the state song, "Maryland, My Maryland" (it remained the official state song until March 2021). The song's lyrics urged Marylanders to "spurn the Northern scum" and "burst the tyrant's chain" – in other words, to secede from the Union. Confederate States Army bands would later play the song after they crossed into Maryland territory during the Maryland Campaign in 1862.
After the April 19 rioting, skirmishes continued in Baltimore for the next month. Mayor George William Brown and Maryland Governor Thomas Hicks implored President Lincoln to reroute troops around Baltimore city and through Annapolis to avoid further confrontations. In a letter to President Lincoln, Mayor Brown wrote:
It is my solemn duty to inform you that it is not possible for more soldiers to pass through Baltimore unless they fight their way at every step. I therefore hope and trust and most earnestly request that no more troops be permitted or ordered by the Government to pass through the city. If they should attempt it, the responsibility for the bloodshed will not rest upon me.
Hearing no immediate reply from Washington, on the evening of April 19 Governor Hicks and Mayor Brown ordered the destruction of railroad bridges leading into the city from the North, preventing further incursions by Union soldiers. The destruction was accomplished the next day. One of the men involved in this destruction would be arrested for it in May without recourse to habeas corpus, leading to the ex parte Merryman ruling. For a time it looked as if Maryland was one provocation away from joining the rebels, but Lincoln moved swiftly to defuse the situation, promising that the troops were needed purely to defend Washington, not to attack the South. President Lincoln also complied with the request to reroute troops to Annapolis, as the political situation in Baltimore remained highly volatile. Meanwhile, General Winfield Scott, who was in charge of military operations in Maryland indicated in correspondence with the head of Pennsylvania troops that the route through Baltimore would resume once sufficient troops were available to secure Baltimore.