I don't think I remember the book where I read this, but it was something about a former chief of police of New York, I believe in the 90's, when criminality was very high, and instead of focusing on the "big crimes", he focused on the small ones, like stopping graffiti on the walls, people jumping the gate at the subway, and other small crimes like that.

The point was to dramatically reduce this "ecosystem" of criminality. I believe it's kind of like when people see garbage on the ground, it's much easier for them to throw their garbage on the ground, too, because "others are doing it". The same thing must be with small crimes and with the people joining the drug cartels.

I believe The District was inspired from this guy's work: http://en.wikipedia.org/wiki/The_District