He's not yet finished picking, but President-elect Donald Trump already has named three retired generals to top posts, raising questions as to why there will be so much military brass in cabinet-level jobs.

Trump on Wednesday named retired four-star Marine general John Kelly to head the Department of Homeland Security, which oversees several critical areas including immigration and border control -- signature issues for Trump.

If confirmed by the Senate, Kelly would join retired Marine general James Mattis as defense secretary and retired Army lieutenant general Michael Flynn as national security adviser. Mattis also needs Senate approval.

While the men bring broad depth of knowledge to the cabinet and considerable expertise, some worry their numbers threaten a cornerstone of American democracy -- that civilians control the military and the government.

"If you have a significant number of (former military members) in your cabinet, you begin to bring into some question whether you actually are maintaining full civil control of a nation," David Barno, a retired general who once led the US war effort in Afghanistan, told AFP.

Yet another retired general, David Petraeus, has been floated as a possible pick for secretary of state, and retired admiral Michael Rogers has been rumored to be in the running for director of national intelligence.

"One more three or four-star general given a senior appointment, and we can start referring to a Trump junta rather than a Trump administration," retired Army lieutenant colonel and military scholar Andrew Bacevich told Time magazine.

Trump was outspoken during his campaign against the generals currently serving under President Barack Obama's administration, boasting he knew more about the Islamic State group than they did, and claiming the White House had reduced the four-stars to "rubble."

- Impressed by generals' 'gravitas' -

One possible reason Trump is drawing so deeply from the military well now is because officers typically keep their political views private, meaning few generals were openly critical of Trump's divisive campaign, while a slew of experienced lawmakers and civilians sharply disagreed with him.

Trump also was likely wowed by the breadth of experience he had seen in the generals, Barno said.

He "is very impressed by the seriousness and the experience and the gravitas that a number of retired generals have brought into interviews with him," he said.

"They are very different kinds of people from many of the people he has surrounded himself with for the other parts of his career, to include through most of his campaign."

Timothy Hagle, who teaches political science at the University of Iowa, said the generals have qualities Trump appreciates: an ability to be "straight talking, very blunt" and to "focus on the mission regardless" of political leanings.

A major concern about a military-heavy cabinet is that Trump might begin to see all world problems through a military prism without giving sufficient voice to other forms of influence, such as diplomacy.

But retired major general Charles Dunlap, who formerly was a top Air Force lawyer and who now teaches at Duke University, said the opposite was often true -- military members who understand the horrors of war are less likely to be hawkish than civilian leaders.

"Retired generals don't clamor for war; they are typically the voices urging that all other avenues be exhausted before turning to force," Dunlap said in an opinion piece published on Vox.com.

How hawkish the new Trump administration will be remains to be seen, but the incoming president has already upset China by taking a call from Taiwanese leader Tsai Ing-wen, breaking with decades of tradition.

And Mattis, who is highly regarded by politicians of many stripes, is outspoken on Iran and has publicly called it the "single most belligerent actor in the Middle East."

Having retired in 2013, Mattis needs a special waiver to serve, as US law bans uniformed military officers from being secretary of defense for seven years after leaving active duty.

Barno, now based at American University, echoed Dunlap's view that more retired generals don't necessarily equate to more foreign military interventions.

"The military leaders that (Trump) has selected will bring a very sober voice about the risks of using military power," he said.

"So I think the military people will bring a cautionary note to a number of these campaigns, in some cases more than some civilian leaders might in the past."