However, in the US, cops today do serve as a domestic military force. Therefore, they're not civilians.
The only way cops can be civilians is if they were de-militarized.
The fundamental problem in the US is that cops are still subject to the civilian justice system in spite of their military role, and that creates problems.
I looked up civilian and military in several dictionaries and civilian does not mean 'not military'. Picking Merriam-Websters as an example, a civilian is, " a person who is not a member of the military or of a police or firefighting force". They all agree that military has to do with armed forces.
"How do I know what I think 'till I hear what I say?"
“looking with fondness towards a reconciliation with Great Britain…I…would rather be dependent on Great Britain, properly limited, than with any other nation on earth or on no nation”. Thomas Jefferson; August 25 1775