It was never going to be easy for Obama to watch Robert Gates walk out of the Pentagon and back into private life. Over two and a half years, Gates has provided impenetrable political air cover for a series of extremely difficult national security decisions, from the surge in Afghanistan to repeal of Don’t Ask, Don’t Tell. Gates’ expertise on military matters and his willingness to be a team player probably represent the greatest influence of any cabinet member on Obama’s first two years in office. From a political point of view, he’s irreplaceable.

But he needed to be replaced.