The shooting of 12 police officers in Dallas on Thursday suggests spiraling violence: The cops were shot during a protest against the shooting of black men by police. A vicious circle of retribution would be something new for the United States, where, unlike in other developed countries, killings by police far outnumber officer deaths in the line of duty.
The point that police kill more people in the U.S. than in European countries has often been made. It's intuitively understandable: American cops have to deal with armed criminals more often because guns are more widely available, and the dominant culture is pro-gun, so people have less of a problem using weapons. For all that, however, relatively few officers get killed.
Federal Bureau of Investigation statistics of "justifiable homicide" by law enforcement officers indicate that in 2010-14, the average number of fatal shootings by police was 428 per year (the number has been hovering around 400 for much longer than that). Also according to the FBI, about 50 officers per year are killed in the line of duty. That's already a rather high ratio of inflicted to suffered casualties — and it disregards the insufficiency of the "justifiable homicide" data; The Washington Post, for example, calculates that a total of 965 people were fatally shot by police in 2015.