Arsenals & Armories are the home(s) of the "well-regulated militia", including the US Army Reserve, the (state-by-state) National Guard, et al.
Many of these buildings are very stylish, very old, and have since been re-purposed to other civic functions.
And, happily, many of them have shown-up on postcards.
First up: The Illinois State Arsenal (1903-1934).http://sangamoncountyhistory.org/wp/?p=1980
... an admirably well-written two-pager on the arsenal-then-armory