The modern USAF formally began operations on September 18th, 1947 following the conclusion of World War 2 (September 2nd, 1945) but maintains a history as far back as 1907. Prior to 1947, the USAF operated as the "United States Army Air Forces" (USAAF) and the service contributed to both World War 1 (1914-1918) and World War 2 (1939-1945). The modern United States Air Force has seen major combat actions since the Korean War (1950-1953) and fields a variety of aircraft to cover all manner of mission types.
Formally, USAF rank abbreviations are not displayed with an ending period. Additionally, the USAF does not maintain a First Sergeant rank common to other branches of American service. Instead, the rank is denoted by the special addition of a diamond within the insignia and observed as a duty / job title. Similarly, the diamond is replaced by a star for the Chief Master Sergeant and Command Chief Master Sergeant ranks.
NOTE: The Five Star rank is generally reserved for wartime only and can also be used as an honorary rank.