Posted By:
steve fMy guess.
Union Army c1864.
The Command Staff, the fellows in the long black coatees indicate Flag Officers. There were hundreds and hundreds of Generals in both Armies.
A security regiment to accompany and protect the Presidential or high command entorage. Lincoln often visited battle sites for briefings.
Formal dress white belts, not a normal work uniform which suggests a Band or Parade Detail/color guard for ceremonies/burials/dedications. Lack of weaponry and abundance of US flags. At least six flags are showing.
I was also considering the first Army Navy contest (white pants) and a West Point cadets' outing of some sort, but the personnel appear older.
If I was in the Army back then, this is the command I would wish to be assigned to. I particularly like the RHB warming up. This card is a beauty.