This item came from my father's collection, he was stationed in Germany shortly after WW II. Items have been in clean storage in a closet ever since. 

From Google Lens:

This item is a vintage United States Army 21st Corps (XXI Corps) shoulder patch from World War II. It features a square design with an olive drab border, a blue cloverleaf, and a red acorn at the center.

Conflict: World War II (1939-1945).  Dimensions: Approximately \(2.75\) inches or \(2.5\) inches square.

History: Activated in December 1943 at Camp Polk, Louisiana, and served in the European Theater of Operations.