President Abraham Lincoln took the step of declaring martial law and suspending civil liberties, including the right of habeas corpus, in Maryland after a riot and subsequent destruction of transportation systems in April 1861. Lincoln had ordered a group of Massachusetts volunteers south to protect Washington D.C., but in Baltimore they were attacked by an angry mob of Southern sympathizers, leaving casualties on both sides. The governor of Maryland and mayor of Baltimore then ordered the destruction of rail lines so that more Union troops could not enter Baltimore. Soon after, Lincoln imposed martial law upon the state.
One reason that Lincoln felt that this step was necessary was because Maryland was a crucial border state between the North and the South. If Maryland seceded, the nation's capital of Washington D.C. would be effectively surrounded by Confederate enemies. Lincoln also was concerned about the protection of transportation routes and telegraph lines, which were vital to the Union army, as it passed through Maryland to confront the South.
Lincoln had a very significant reason for imposing martial law on the state of Maryland. When Virginia seceded from the Union, Maryland became a very important state for the Union. Since Washington, D.C. was composed of land donated by Virginia and Maryland, it was imperative that Maryland remain in the Union. If Maryland were to secede, Washington, D.C. would have been cut off from the Union. The capital of the United States would have been in another country! Thus, by imposing martial law, Lincoln was able to arrest those legislators who were going to vote for secession. As a result, Maryland didn’t secede and remained in the Union. Martial law was imposed to save the capital from being in enemy hands. It wasn’t the only time Lincoln did something questionable in order to save the country.