It’s called the Broken Windows Theory. It is popular in both criminology and politics, and says simply that the physical state of a neighborhood affects the possibility of crime. Let one broken window go unfixed, the theory goes, and soon there will be another broken window, and another, then an escalation, trespassing, litter, then criminals are emboldened and more numerous and work under the theory that in this neighborhood, nobody really cares. And down it goes.
It makes even small crimes important, minor issues like vandalism crucial. It means the proliferation of gang graffiti is far too important to ignore. Unerased, what looked like mischief is elevated to threat.