WASHINGTON — If retired Gen. James Mattis is confirmed as President-elect Donald Trump's first secretary of defense, he will take over arguably the largest bureaucracy in the world.

The question is: Who becomes his right-hand man?

The current holder of that role, Deputy Secretary of Defense Bob Work, believes that before Trump and Mattis fill the deputy spot, they should decide what role they want the deputy to have. In a Dec. 4 interview, while returning from the Reagan National Defense Forum, Work described four general types of deputies, each defined by the relationship with their boss.

The first kind of a chief management officer, someone like [David Packard, deputy from 1969-1971] – "a person from business comes in, knows how to downsize and go after efficiencies," Work said. There is also the policy-heavy model, such as that held by Paul Wolfowitz under Donald Rumsfeld, whose job it is to help the secretary with "knotty policy problems."

A third model is the "alter-ego" model, one Work said was used with Secretary of Defense Leon Panetta and his then-deputy, Ash Carter, where the deputy acts essentially as an extension of the secretary, filling in wherever needed. And finally, there is the CEO/chief operating officer model that Work said he and Carter, now in the top job, have adopted.

While emphasizing that Trump and Mattis are going to have to make the deputy decision among themselves, Work did offer a summary piece of advice.

"They should pick the person for the job they want the deputy to do," Work said of the next administration. "I can’t tell you how he should have a deputy, but there are four different models, and you should pick the best athlete for the model that you want. You shouldn’t pick a Paul Wolfowitz, a policy guy, and say I want you to be [a] Packard."

Analysts are divided on what model might work best for Mattis, whose position as the first retired general to take the secretary role since George Marshall could complicate his relationship inside the building.

Mackenzie Eaglen, a former Defense Department official now with the American Enterprise Institute, says that the policy-wonk model "does not work. Plain and simple."

Because the secretary also does policy and the undersecretary of defense for policy is effectively the number three spot in the building, "it is useless and a waste to have that also as the number two," Eaglen said. "I want competent management of the defense enterprise. Alter-egos are fine but not necessary for total success; more like gravy."

"Mattis would be best served by a combo of the old Pentagon hand and a COO. If there is going to be any sort of possibility of a buildup, someone has to be there running the trains faster than ever. They cannot start from behind. Trump will want and expects results, quickly," Eaglen added.

Loren DeJonge Schulman, an analyst with the Center for a New American Security who served both in the Pentagon and on the National Security Council for the Obama administration, called Mattis’ choice of deputy one of the key relationships the next secretary will have to manage, on par with the chairman of the Joint Chiefs, the national security adviser and President Trump himself.

"Key to that relationship will be a clear-cut and transparent division of labor between the two and their staffs - and the trust needed to back it up," Schulman said. "The most important quality for Gen. Mattis's deputy will be trust — trust between those two and the ability to trust and be trusted by stakeholders in the building and on the Hill. The next [deputy] will have the opportunity to be bold and take risks, but won't succeed unless perceived as both fair and empowered."

Schulman believes the next deputy will have to oversee what is expected to be a notable increase in budget and force structure for the Pentagon — a major challenge even if it is good news for the DoD.

"The services may be inclined to use bigger budgets to delay tough choices they need to make regardless of the resourcing picture, or to grow forces and platforms unsustainably for the long term," she said. "The Hill will also have strong views on whether and how to apply such a new spending plan. The new deputy will need to be empowered by the secretary to run a programming process that addresses those risks up front."