The constituent states of the United States of America were concerned backed in the 18th century, that if the state militias could be regulated, then if it so chose, only the Federal Government would be able to put down slave rebellions. Read this, it is fairly convincing.
The real reason the Second Amendment was ratified, and why it says "State" instead of "Country" (the Framers knew the difference - see the 10th Amendment), was to preserve the slave patrol militias in the southern states, which was necessary to get Virginia's vote. Founders Patrick Henry, George Mason, and James Madison were totally clear on that ... and we all should be too.
If the anti-slavery folks in the North had figured out a way to disband - or even move out of the state - those southern militias, the police state of the South would collapse. And, similarly, if the North were to invite into military service the slaves of the South, then they could be emancipated, which would collapse the institution of slavery, and the southern economic and social systems, altogether.So the Second Amendment came about. The Second Amendment is now read to mean an unqualified right of individuals to bear arms. And the result is periodic havoc. So Americans needlessly die today because of their legacy of slavery. Karma is a bitch.
These two possibilities worried southerners like James Monroe, George Mason (who owned over 300 slaves) and the southern Christian evangelical, Patrick Henry (who opposed slavery on principle, but also opposed freeing slaves).