Useful perspective on issues surrounding the nomination of the retired Marine Corps general James Mattis to be Secretary of Defense, including the issue of civilian control of the military, can come from reflecting on the career of the one other general ever to be U.S. Defense Secretary.
Whether the appointment of Mattis turns out to be good or bad will depend as well on other things, but for comparison and context, consider the role and talents of the third Secretary of Defense, George C. Marshall. (After World War II, a reorganization transformed the Department of War, which had existed since 1789, into the Department of Defense.)
Marshall had a career as an Army officer but, apart from 18 months as a second lieutenant of infantry during the insurgency in the Philippines that followed the Spanish-American War, he rose to five-star general without ever commanding troops in combat. He instead was a brilliant planner and organizer.
During World War I, he was a staff officer who was heavily involved in the planning of operations for the American Expeditionary Force. As Army chief of staff throughout World War II, Marshall could be said to have managed the enormous allied war effort as much as any one person did. This was one of two roles that earned him a distinguished place in history.