Maryland was a slave state. President-elect Lincoln was almost assassinated in Baltimore on the way to Washington, DC.
Yes, Maryland was a slave state, and Lincoln was determined that the nation’s capital would not be surrounded by the Confederacy. So he dragged Maryland into the Union when there were many here who wanted to side with the CSA.
You can see that many Marylanders were not happy about this when you look at the lyrics of our state song:
http://msa.maryland.gov/msa/mdmanual/01glance/html/symbols/lyrics.html