Adm. Mike Mullen, now serving as the Pentagon’s top officer as Chairman of the Joint Chiefs of Staff, is expected to retire after his term ends in September, and Gen. David Petraeus, commander of all U.S. and NATO troops in Afghanistan, is under serious consideration to become the next head of the CIA when he leaves the military behind.

But possibly the hardest part of the shifting White House cabinet and military leadership will be naming a successor for Defense Secretary Robert Gates, Reuters reports in an analysis piece.

After five years, Gates has announced his intention to retire this year.

Who’ll take his spot?

Read Analysis: Is Obama ready for Pentagon leadership turnover? on Reuters.com