Why did President Lincoln call for martial law and suspend citizen's rights in the borders states during the Civil War?

Abraham Lincoln, was desperate to prevent Maryland from joining the Confederacy (by force, if necessary), as such a turn of events would surround the Union capital of Washington, DC with hostile territory. The day after "Maryland, My Maryland" was first published, Lincoln suspended the writ of habeas corpus in Maryland. Subsequently, as Virginia's secession from the Union became imminent, Lincoln sent the Union Army to occupy Annapolis and Baltimore, and declared martial law.