Task: Question Answering
Input (English): Context : Karl Gratz (24 January 1919 -- 14 March 2002) was an Austrian born Luftwaffe fighter ace and recipient of the Knight's Cross of the Iron Cross (German: Ritterkreuz des Eisernen Kreuzes) during World War II. The Knight's Cross of the Iron Cross was awarded to recognise extreme battlefield bravery or successful military leadership. 
Question : What was the military branch for Karl Gratz?
Output (English): Luftwaffe