Radley Balko summarizes the problem with militarizing our police force:
Once again, cops aren’t soldiers. American cities aren’t battlefields. And U.S. citizens aren’t potential combatants. This isn’t pedantry. It’s about the mentality with which police officers approach their job, and about what sort of relationship they’re going to have with the people whose rights they’re supposed to be protecting.
If you haven’t read it, Balko’s book/paper Overkill: The Rise of Paramilitary Police Raids in America is crucial reading for understanding contemporary America.
See also: US as Police State