There should be no resson for military aircraft, or any other aircraft for that matter, to be in flightpaths of civilian aviation routes unless there is an intervention event. Proximity of two very different agendas in the same theatre is simply a bad idea. Manage the risks, not just the protocols, to reduce tragic events like this.
In industrialized workplaces, every day, we say that “Accidents don’t happen, Risky behaviours happen”.
This army helicopter came from some base far upriver, and was going to somewhere downriver on a training flight, not to Reagan but to just do some urban environment training with NVGs on.
Apparently military helo fliights like this one are commonplace. It is not just mil aircraft using Reagan airport, but just coming and going from their own bases. (I saw an interview with a military helo pilot, who descrived flying with NVGs in such an area, and the sheer glare of lights making it hard to actually see individual objects clearly.)
And above DC, there's always fighters roaming around that area (since 9/11), but they keep out of the way of what goes on below unless they need to "intervene" for an incident.
I've heard it said that Reagan airport is one of those airports that would likely not be built today, considering where it is, with such busy & tight urban airspace. Probably the airport of choice for DC big shots, but other than them, it is really not a great place for major airlines to come and go.
(Funny fact: As a youth I once flew from that airport to NYC, on Trump Airlines.

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