I find the historical marker below on the campus of the United States Naval Academy in Annapolis, Maryland really interesting. It isn't necessarily well known that owning slaves was legal in Maryland when the Civil War broke out in the United States. However, Maryland and a few other "slave states" did not secede from the United States of America (the Union) to join the Confederate States of America. (Missouri was another example of a slave state which did not secede.)
If you don't know your geography very well, you may not realize that Maryland is immediately west, north, and east of Washington, DC. Virginia which was part of the Confederacy was to the west, south, and east. Therefore, if Maryland had sided with the Confederacy in the Civil War, President Abraham Lincoln and the United States Capital in Washington, DC would have been surrounded by the Confederacy.
Based on the historical marker below, it appears that the Union did not want to allow this unpleasant possibility to occur. Union troops took over the Maryland capital of Annapolis and physically prevented the Maryland Sate Legislature from convening and even considering a motion to secede from the Union.
I'm not advocating this should have happened... I'm glad we no longer allow slavery. However, just imagine how United States history might have been different if Washington, DC had been surrounded by the Confederacy at the beginning of the Civil War.