https://www.washingtonpost.com/news/morning-mix/wp/2014/08/1...
I think the biggest issue with police forces is that they're all basically trained to treat all situations with equal gravity. They fear for their lives even when it is not warranted. And because of that they respond more like a military force, with a level of violence out of all proportion to the situation.
The police are a bigger deterrent to peaceful protest in this country than any politician, and they have been since they came into existence.