In 2012 the government of El Salvador made a secret deal with gang leaders. The gangs promised not to kill as many people and in return the government agreed to transfer the gang leaders from the only maximum security jail in the country that is not controlled by gangs into other jails, all of which are controlled by the inmates.
The homicide rates in the country immediately fell by a whopping 60%.
But the deal didn’t remain secret for long. Obviously, everybody found out. Gangsters don’t tend to be good at keeping these things to themselves, and it was pretty obvious something was on when the scariest gang leaders massively left the maximum security jails.
The media worked hard to spread the message that the deal wasn’t working and that homicide rates actually grew. This was provably and obviously untrue but it’s very easy to convince the public of pretty much anything.
There was a huge backlash against the deal. The government broke the agreement with the gangs. Everybody who had negotiated the deal got fired. And homicide rates immediately went back up.
In the short time it was in place, the deal is calculated to have saved the lives of over 6,000 people.
This story was extensively covered by journalist Roberto Valencia, among other people, if you wonder about sources.