Complex sentence: He re-entered the Navy as Chief Resident in Medicine at the Navy Medical Center in Oakland, California, becoming Board-certified in Internal Medicine in 1993 and in 1996, he was appointed Fleet Surgeon to the US Navy's Third Fleet.
Split into simpler sentences: He re-entered the Navy as Chief Resident in Medicine at the Navy Medical Center in Oakland, California, becoming Board-certified in Internal Medicine in 1993. In 1996, he was appointed Fleet Surgeon to the US Navy's Third Fleet.