Secretary of Defense Ash Carter is a brilliant man. He has a Ph.D. in theoretical physics, and is well known in scientific and technological circles. He served three stints in the Defense Department: assistant secretary of defense for international security policy in 1993-1996; under secretary of defense, with responsibility for procuring all technology in 2009-2011, and deputy secretary of defense in 2011-2013.
Now to the email timeline.
On October 6, 2014, Carter was appointed a Distinguished Visiting Fellow at the Hoover Institution of Stanford University and the Payne Distinguished Visitor at the Freeman Spogli Institute for International Studies. His office was located on the first floor in the Lou Henry Hoover Building (just a few doors away from my office). Above him on the second floor are the offices of Hoover's cyber security program headed by Dr. Amy Zegart, which is part of Stanford's wider $15 million cyber security program.
On December 5, 2014, President Obama nominated Ash Carter for secretary of defense. He was confirmed by the U.S. Senate on February 12, 2015.
Dr. Zegart was instrumental in arranging a White House Cyber Summit at Stanford on February 13, 2015, at which President Barack Obama gave the keynote address.
In the wake of the presidential summit, Carter returned to Stanford on April 23, 2015, to unveil the Pentagon's new cybersecurity strategy. He called for a renewed partnership with Silicon Valley to create an unbeatable pact between military strategy and technologists.
On December 16, 2015, the New York Times reported that Ash Carter conducted a portion of his government business using a personal email address in direct violation of the department's rules. In response to the story, a defense department spokesman said, "After reviewing his email practices earlier this year, the secretary believes that his previous, occasional use of personal email for work-related business, even for routine administrative issues and backed up to his official account, was a mistake."
If it's all right for the secretary of state to use personal email in the conduct of government business, it must be all right for the secretary of defense, even though it's against departmental rules.
One wonders how widespread the use of personal email addresses is throughout the highest echelons of the federal government.