WWII US Army AAA (Anti-Aircraft Artillery) Units have some interesting history. AAA units were in use during WWI, and the improvements in air power solidified their need for national defense. In the post-Great War reorganization of the US Army in the early 1920's, multiple units were created in the National Guard (NG) and Organized Reserve (AR) for this purpose. Most NG units were descendant units of old regimental units with the new technologically advanced anti-aircraft artillery role added as their mission. Due to the separation of responsibilities of US Defense, these units fell under the Coast Artillery Corps, and were some of the first units activated in the 1940-1941 call-up for the "limited national emergency." A little known unit was the 200th Coast Artillery Regiment, of the New Mexico National Guard. They were called up in 1941 and headed to the Philippines later that year, stationed around Clark Field, later taking part in the defense of Bataan and ultimately the Bataan Death March.
If the subject interests anybody, there is a great website with histories of the Coast Artillery AAA units at this link:
http://cdsg.org/coast-artillery-corps/