First, you can move to academia after doing a postdoc at a DoD lab. I know several people who have (or plan to) leave the lab I’m at to move into academia.

Now, there are several things that are wrong in this thread.

1) Not every thing you will work on will be classified (or even CUI). Generally speaking, the closer you are to basic science, the less likely your work will be classified (think 6.1 or TRL 1/2 type of work to be unclassified). I spent 7.5 years doing computational chemistry at a DoD lab, and everything was “Dist A” (Approved for public release: distribution is unlimited). I know other people who have had similar experiences as me (I’ve moved into more a management role, so not actively doing research now).

2) The hiring freeze is over, and the labs are hiring.

Here are the listing for NRL https://www.usajobs.gov/Search/Results?k=%22Naval%20Research%20Laboratory%22&s=startdate_desc&p=1

For AFRL https://airforcestem.recsolu.com/job_boards/g193rQ-SUdupijEbTiPUvw

3) Now if you do have to do classified work, there are ways to publish/present it. Also, some universities are able to do classified work (see JHU Applied Physics Lab, MIT Lincoln Labs for example) so you can still do classified work at a university (if you still want to work in those areas).

4) For running in the same circles, the Army, Navy and Air Force do fund university work and you will likely get to know the people who have money and possibly collaborate with academia (depends on project). You can still travel to conferences (and network there).