To paraphrase Tip O'Neill, "All policing is local." But that doesn't mean that something in one town can't have an effect on policing nationwide. And a trend can be large and worrisome -- and national -- without being universal. That's why they call it a trend.
I don't know what's going on everywhere (or even most-where), but I can tell you a bit about Baltimore. And I suspect it holds true in many cities.
I looked calls for service, arrest numbers, and crimes. Most dramatic is the drop of arrests in Western District. I looked at arrests with a post number assigned to it. The majority of arrests by patrol officers are discretionary. These are the ones I presume were not being made. Arrests listed without a post (a geographic beat) would be a specialized unit that didn't know or care about the post, a court arrest, or a probation violation. Arrests with no post listed also declined, but not nearly as much.
Arrests in the Western District, from May to December, were down a whopping 47 percent comparing 2014 and 2015 (39 percent overall in the city).
Look, the link between police and crime prevention will always be shrouded in some mystery. Causation in the real world can never be "proved" with certainty. But at some point, if you get enough correlation and no alternative causation, correlation might actually be indicative of causation! [queue stats-class thunder bolts.]
Now there are good and not so good reasons for this drop in arrests. But leaving that why: it happened. Police were less involved, by choice and necessity, and violence skyrocketed. Just because correlation does prove causing, correlation certainly doesn't mean causation is impossible or even unlikely. I mean, what else changed in the Western except police and crime?
Arrests and crime vary a lot during the year. Winter is slow. Late spring and late summer hot. But the drop in Baltimore arrests began before the riots of April 2015. They started going down in July and August of 2014, after the deaths of Eric Garner (Staten Island) and Michael Brown (Ferguson). That's when attention turned to police. That's when officers started feeling they were being targeted, not for malicious actions but for trying to do good. And there's a huge drop is arrests in December of 2014 (when protests really got going). Just 2,126 citywide (probably the lowest monthly figure in 50 years). December 2014 was 28 percent off 2013.
And then May 2015, when normally you'd see more arrests as the weather warms and kids get out of school, arrests were down 50 percent from the previous year. (I looked at arrests that have a post number listed in Open Baltimore data. I can't be 100 percent certain, but I think these are more likely to be on-view arrests from patrol officers and in response to calls for service. Arrests without post number are more likely to be specialized units and administrative arrests.)
Cops stopped making discretionary arrests and being proactive in clearing corners and frisking subjects. Look, it's no surprise where shootings happens and who gets shot. There is a criminal class in Baltimore. You can police aggressively or wait for somebody to call police. Even then, when responding, you can get out of your car or not risk "harassing" the "innocent" youth.
Arrests, especially non-domestic misdemeanor arrests, are a good proxy for discretionary officer interaction with the public. Arrests can also tip the crime stats up because many crimes aren't recorded unless an arrest is made (which is why, as an indicator of crime overall, I trust shootings and homicides more than anything else). In 2015, arrests in the Western District went down from 215 April to 114 in June (and an outlyingly low 79 in May, when cops were busy with post-riot curfew). The previous year, 2014, saw 259, 292, and 265 arrests in April, May, and June, respectively. To put this in perceptive, my squad (one of three working midnight and one of nine total in the Eastern District) used to make 60 arrests a month on average.
Meanwhile, after the riots, with police demoralized and understaffed and politicians wasting resources prosecuting innocent cops, criminals in the Western were shooting or killing another black man every other day. These deaths are real. They are evidence. And they matter.