I cannot believe that Hagel is the first combat veteran to hold the office of Secretary of Defense.
For the record, the title of Secretary of Defense only goes back to 1949 (before that it was the Secretary of War or individual Sectaries of the Army and Navy), and its a lot rarer than I expected for the position holder to be a military veteran than a business management type, but at least one and maybe two previous Secretaries of Defense would qualify as “combat veterans”: (1) Elliot Richardson served as Secretary of Defense under Nixon (before serving as Attorney General). He was a decorated combat veteran in WWII and served in the Normandy invasion. (2) Casper Weinberger served in the Pacific in WWII, although he served in intelligence so it is not clear whether he was ever in front line combat.