One big problem in his appointment, its against the law (1947 National Security Act). He has to have been off the active list for 7 years before he can be considered. That regulation has been by-passed once, when George C Marshall was made SecDef in 1950, but he had already been in government as SecState and the appointment was driven by the fact that the US was involved in a major war (Korea) and Truman needed somebody trusted to keep his egotistic field commanders in line (which Marshall was known to be capable of doing).