Let’s start with the likely departure this summer of Defense Secretary Bob Gates. A Republican holdover who manages the Pentagon by a combination of wisecracks, sharp policy analysis and the occasional firing of subordinates, Gates has been one of the most effective defense secretaries in recent decades. It’s hard to imagine someone lacking his long experience (and that includes every likely candidate) playing as effective a role.

The same hard-to-replace problem exists with Adm. Mike Mullen, chairman of the Joint Chiefs of Staff. With his doughy Karl Malden face and his rambling syntax, he’s an unlikely fit with Obama. But he has led the military family with finesse — anticipating problems and offering skeptical judgments on recommendations from the military brass and politicians alike. He also led the military in its historic acceptance of gays in uniform, an issue he saw as a matter of personal conscience rather than political expediency.

Mullen’s likely replacement is Marine Gen. James Cartwright, a kind of Obama-in-uniform — dry, cerebral and brilliantly analytical but lacking the warm and fuzzy quality. Where Mullen is open and easy, Cartwright can seem closed and tight, according to several of his colleagues. Still, says one White House official of Cartwright, “He doesn’t open his mouth without saying something impressive.”

Long shots for chairman include Army Gen. Martin Dempsey, who is similar in style to Mullen, and Air Force Gen. Norton Schwartz, who gets raves for leading the Air Force into a new era as chief of staff. Adm. James Stavridis, the NATO commander, is seen as a future chairman, but probably not this round.

I took a flier a month ago and predicted on a Sunday television show that Gates would be replaced by CIA Director Leon Panetta, who would in turn be replaced at the agency by Gen. David Petraeus. It was clear at the time that Panetta was Gates’s favorite, but the White House has now embraced him, too. As a former congressman, budget director and White House chief of staff, Panetta has the right mix of skills for cutting the Pentagon budget, an important challenge for the next SecDef. The only worry at the White House is that Panetta is sometimes accident-prone in his public comments. Another name being discussed is Navy Secretary Ray Mabus.

The White House has gotten intrigued (pardon the pun) by the idea of Petraeus as CIA director. Though he’s a career military officer, Petraeus has worked closely with the agency in Iraq and Afghanistan. He has a knack for back-channel contacts and would be an effective liaison with foreign governments. I also suspect that Petraeus would like the job. But does Obama trust him sufficiently to give him such a sensitive position? We’ll find the answer in a “dead drop” on Pennsylvania Avenue.

Few would have predicted it two years ago, but CIA career officers will miss Panetta if he goes. He has won Langley’s loyalty by fighting for the agency in disputes with Congress and the White House. The CIA likes directors it is able to co-opt into the agency’s culture, and, for better or worse, that includes Panetta. The prospect of Petraeus as boss makes some CIA officers nervous. But he might shake up the place in good ways.

All the new candidates for these jobs have their strengths. But it’s hard to see them operating together as smoothly as the team that’s breaking up. What made this one work was that the principal players — Gates, Mullen, Panetta — had all been in senior positions for years and didn’t have the sharp-elbows problem that has been the ruination of so many national security teams in the past.

Can Obama find a new team that maximizes utility once again? He’s looking hard, and he has a solid roster of candidates. But finding smart, tough people who can also play well with others is never easy — and Obama is likely to end up with a strong but more fractious crew.

davidignatius@washpost.com