Actually the 82d Airborne folks who went to DC in 1968 had a high number of combat veterans in it. I believe they had returned from Vietnam in the previous 6 months. I recommend the chapters about the 1968 riots in this Army History that I was involved in researching.
Dr. Paul J. Scheips,
The Role of Federal Military Forces in Domestic Disorders, 1945-1992 [30-20] https://history.army.mil/catalog/pubs/30/30-20.html
https://history.army.mil/html/books/030/30-15-1/CMH_Pub_30-15-1.pdf
Certainly, also true in 1969. A draftee could go through Basic/AIT, Airborne School, and Vietnam and still have between 4-6 months still left when they returned from RVN. I was a no CIB 2LT amongst a bunch of combat veterans. The Army noticed and made sure that I got one.
Thank your for those links for later. I seem to recall reading (I was only 8 at the time) that they had machine gun nests with .50 cal set up at various intersections around DC. Am I remembering correctly - or is that just a rumor?
Or - perhaps just sand bag bunkers for security - but no machine guns.