Robert Gates, appointed Secretary of Defense by George W Bush in 2006, continued to serve in that office under President Obama until he retired in 2011. Gates has also served as an Air Force officer, as Director of Central Intelligence under George H.W. Bush, and as an employee of the CIA and the National Security Council as well as president of Texas A&M University. A Washington Post review of his memoirs said that he was "widely considered the best defense secretary of the post-World War II era."
According to a piece published yesterday at Politico (link) summarizing his appearance in MSNBC's program Morning Joe, Gates does not have a high opinion of those seeking the Republican nomination for president. "First of all they, they don’t know what they’re talking about,” he's quoted as saying.
On Donald Trump's threat to "bomb the shit" out of ISIL and Ted Cruz's advocacy of "carpet bombing," Gates said, "Carpet bombing would be completely useless. It’s totally contrary to the American way of war" because it costs huge numbers of civilian lives. "So, I mean, part of the concern that I have with the campaign, particularly when it comes to national security, is that the solutions being offered are so simplistic and so at odds with the reality of the rest of the world, with the way the world really works."
He also suggested that those without political experience (such as Trump, Carson, and Fiorina) would not have the skills needed to be effective presidents.
In a separate interview with Bianna Golodryga reported yesterday by Yahoo News (link), Gates also criticized President Obama's foreign policy. Concerning his experience with Hillary Clinton when she was secretary of state, Gates said, "During the period I was there, I found her a good colleague: I found her tough-minded, and I found her quite realistic. The first time we really disagreed on anything was on the decision to intervene in Libya in early 2011."