Admiral William Crowe, former Chief of the Joint Chiefs of Staff, bacame ambassador to the United Kingdom following his military service--certainly not a career diplomat, but well qualified.
Another Adimral with extensive experience in the Pacific area, Thomas Foley, was given the ambassadorship to Japan, preceding James Baker.
I worked for Admiral Crowe when he was Commander in Chief of the Pacific (around 1985); and much of his efforts in that capacity involved interaction with heads of state throughout the Pacific. So, though neither he nor Foley were career diplomats, they were very carefully groomed to take on such duties.