Only certain parts of certain cities[0]. It's almost entirely a result of the War On Drugs. In areas where drugs and gangs are a problem, the homicide rate is very high. In wealthy/safe parts of town, violent crime is not dramatically higher than the rest of the developed world.
Does San Francisco have a particularly large amount of "war on drugs" related gang violence? It's still 8x the homicide rate of London. And whether it's caused by the war on drugs or not, the fact remains police still have to deal with it.
[0] http://en.wikipedia.org/wiki/File:Chicago_violent_crime_map_...