According to historian James McPherson, after Fort Sumter and the secession of four states of the Upper South, Maryland "flamed with passion". The State legislature and several counties wanted secession, but the Unionist Governor Thomas Hicks refused to call the legislature into session to forestall action. Baltimore, with a third of the state's population, was an especially chaotic place of pro-Confederate sentiments, with Confederate flag flying in many homes. When the 6th Massachusetts had to pass through the city to reach Washington, they were met with hostile civilians and mobs that attacked them with bricks and pistols. The scared soldiers opened fire, starting a riot that took the lives of 4 soldiers and 12 civilians.

Secessionist, considering this an act of coercion by the "blackest of Black Republican States", sabotaged the railways and telegraphs that reached Washington, cutting the capital from further reinforcements or communications. Many believed that Virginia was poised for an attack. Even Lincoln despaired, declaring that there wasn't any North, while General Winfield Scott organized government clerks into militias, decided to make a last stand if necessary. Fortunately, General Butler and his men managed to repair the railways and pour reinforcements into the city by sea. This military build-up probably discouraged the secessionists when Hicks finally called the legislature into session.

Lincoln had half a mind to send the Army to arrest the members of the Legislature, but he decided against it. Good call, for the Legislature refused to consider secession, though it did denounce the war the Federal Government had "forced" into the South and declared that they wouldn't take part on it. But neutrality wasn't really an option due to Maryland's position, and soon the Unionists asserted themselves, winning an special election for Congress and organizing army regiments. This was because the state and its economy were intertwined with the North. Seceding would cut Maryland's land and water connections, which would be a fatal hit for her economy.

Despite this, Lincoln still directed the Army to arrest and imprison many prominent Secessionists. None other than Roger B. Taney, the Chief Justice, issued writs of Habeas Corpus, but Lincoln suspended the writ, declaring it one of his war powers and simply not listening to Taney. He went farther after the Union's defeat at Bull Run, arresting members of the legislature, the Major of Baltimore, the Chief of Police and many more before they could do anything. Then he mandated new elections, which resulted in a smashing Unionist victory.

Maryland, along with other Border South states such as Kentucky and Missouri, was more lukewarm to secession that Upper South states like Virginia. Maryland's economy was based more in trade with the North than in plantations, and half of Maryland's black people were free. Passions ran high, but at the end Lincoln's actions (which were, indeed, constitutionally questionable), the Union military build-up in the state and Maryland's own connection to the North, all allowed her Unionists to take control and prevent secession.

Sources: Battle Cry of Freedom by James McPherson and The Road to Secession, Part II by William W. Freehling.