>That is the Old Guard, 3rd US Infantry.
I didn't know that until this article today…
His largest unit, the 3rd Infantry Regiment, also known as "The Old Guard," normally supplies the spit-shined guards at the Tomb of the Unknown Soldiers and conduct the Army's most public events. But behind the scenes, the 3rd Regiment reverts to its true infantry origins. With activation of the Joint Task Force, the Regiment has been given responsibility to "conduct homeland defense and civil support operations to defend and secure the National Capital Region."
https://www.newsweek.com/exclusive-washington-dc-faces-coronavirus-spike-secret-military-task-force-prepares-secure-1498276 (paragraph 30)