The game of musical chairs already upsetting the White House starts soon at the Pentagon, where it could become an even bigger headache for President Obama. Not only is Defense Secretary Bob Gates leaving, but so is the Joint Chiefs chairman, Adm. Mike Mullen. And from the looks of things, replacing Gates might be Obama’s easiest chore, with a handful of favorites, including former Secretary of State Colin Powell, on his list. The Joint Chiefs post is different. The easiest and most likely move is to promote the No. 2, Marine Gen. James Cartwright, a White House fave. But that would leave a hole at No. 2. We asked around about giving the Afghan war boss, Army Gen. David Petraeus, the top job, but insiders say that would create a vacuum in Afghanistan and perhaps a potential 2012 challenger to Obama. There’s some talk Mullen might be replaced by another admiral, James Stavridis, head of the European Command, but it’s unheard of to have back-to-back admirals as chairmen.