It is a WWII American Campaign "Hat" pin. The American Campaign Medal was established per Executive Order 9265, dated 6 November 1942, by President Franklin D. Roosevelt and announced in War Department Bulletin 56, 1942. The criteria was initially announced in Department of the Army (DA) Circular 1, dated 1 January 1943, so that the ribbon could be authorized prior to design of the medal. The criteria for the medal was announced in DA Circular 84, dated 25 March 1948 and subsequently published in Army Regulation 600-65, dated 22 September 1948.
The ribbon design was approved by the Secretary of War on 24 November 1942. The blue color represents the Americas; the central blue, white and red stripes (taken from the American Defense Service Medal ribbon) refers to the continuance of American defense after Pearl Harbor. The white and black stripes refer to the German part of the conflict on the Atlantic Coast, while the red and white stripes are for the Japanese colors and refer to that part of the conflict on the Pacific Coast.