I'm not sure the United States has ever had a large-scale successful intervention of the type needed.

The Mafia used to be a substantial problem and a source of a large fraction of the murders in many major cities. This was solved through dramatically increased police intervention and new laws (the RICO act) that allowed people associated with the Mafia to be locked up for far longer on far lesser charges. It actually worked, but this type of "strict laws, big sentences, lots of police" type of intervention seems incredibly unlikely presently. And even then it's not certain to work, since part of the reason the Mafia violence dwindled was that it was largely ordered from the top down; a lot of contemporary violence is more organic.

So perhaps we need a completely different style of intervention. The Harlem Children's Zone has astounding results. Anyone who thinks youth in higher-violence areas are a lost cause should look up HCZ and come back when they've scraped their jaw off the floor. (https://hcz.org/) However, it's highly resource-intensive and has only really proven itself locally. If we could enact something like this widely--wow! But managing something this effective and this audacious nationally would be something that's never been done before, as far as I can tell.

Solving the problem is a very tall order; it's not an area in which the U.S. has displayed competency (not overseas, either). But we have enough precedent to suggest a couple of paths.