WASHINGTON — Ash Carter is on a clock.

It's the widely acknowledged truth about his time as secretary of defense — when a new president takes office in January 2017, likely bringing along a handpicked new leader for the Pentagon.

For Carter, that means any hope of implementing his reform agenda needs to happen quickly, in a building that top DoD officials often complain is designed to oppose efficiency.

But it's worth noting that he does have one advantage in any attempt to shape the future of the Pentagon: Over the course of his roughly two years in office, Carter will have a say in an unusually large number of top positions, including the entirety of the Joint Chiefs of Staff. And that may be where his biggest chance to impact affect the Pentagon lies.

Carter is "looking to the long term, because he understands these people will be around past the administration," a former Pentagon official told Defense News. "People say he doesn't have time to make a huge impact on the DoD. I totally reject that. Just from these personnel decisions, he's going to have a fundamental impact."

To be clear, President Barack Obama has final say on these positions — on background, a number of experts have said they believe the selection of Gen. Joseph Dunford, then Marine Corps commandant, as chairman of the Joint Chiefs was driven primarily by the White House. And the Senate will need to confirm any selections, which is never a sure thing.

But Carter will have a say, leaving those who track the Pentagon looking for signs about who might be on the rise inside the building.

So far, Carter has overseen the nominations of Dunford; Gen. Paul Selva, the head of US Transportation Command, as the vice chairman; Gen. Mark Milley, commander of Army Forces Command, who is nominated as Army chief of staff; and Adm. John Richardson, current head of naval reactors, was nominated to become the next chief of naval operations.

Before the end of the Obama administration, Carter will get a chance to help replace Dunford as Marine commandant and pick successors to Gen. Mark Welsh as Air Force chief of staff and Gen. Frank Grass as chief of the National Guard Bureau — a clean sweep of the Joint Chiefs.

In addition, to filling the Transportation Command hole left by Selva, Carter will have a say on the next secretary of the Army, and perhaps secretary of the Navy if Ray Mabus, who has held the job since 2009, decides to retire.

The former official believes there is no "secret sauce" Carter is using to make his choices, but rather that he is going spot by spot.

"I don't know there is a trend line, other than the very deliberative, thoughtful process which is being used to pick the very best people for the specific job," the former official said.

That said, Andrew Hunter, who served under Carter at AT&L when he was undersecretary of defense for acquisition, technology and logistics (AT&L), believes it is "definitely an advantage" for a candidate to have worked with Carter in the past.

From 2009 to 2013, Carter served first as AT&L undersecretary of defense for Acquisition, Technology and Logistics (AT&L) and then as deputy secretary of defense, jobs in which he would have had contact with a number of top military officials.

Hunter, now the director of the Defense Industrial Initiatives Group at the Center for Strategic and International Studies, believes Carter likes to know the "quality of their thinking" when weighing someone for a new job.

And both Hunter and the former official say that operational experience is important to the secretary as he makes his choices.

Unconventional Picks

Those characteristics come through with Carter's first two choices on the Joint Chiefs, Milley and Richardson, both were seen as surprising choices.

Hunter said Carter's picks are a "solid set" of individuals that speaks to the type of person Carter is looking at.

"To my mind, this is not a random collection of folks," he said. "This looks like what you would expect from Ash Carter — a carefully thought-through group of folks who are not just people he likes, but people he thinks are really going to be able to tackle these jobs they're being assigned to."

One current Pentagon official noted that both men had worked with Carter previously, and he had the chance to "see what shaped them, both in war and in Washington."

Hunter said highlighted the choice of Milley was a good example, even though some view the pick as a found it surprising. Milley was picked over other officials such as Gen. Daniel Allyn, the vice chief of staff; Gen. John Campbell, the top commander in Afghanistan; Gen. David Perkins, the commanding general of Training and Doctrine Command; and Gen. Vincent Brooks, commander of US Army Pacific.

Carter may value Milley not only for his experience in the field but inside the Pentagon, where he served as a military assistant to then-Defense Secretary Robert Gates and later was deputy director for regional operations on the Joint Staff. The former role, though short, gained him a reputation as an able go-between for the Army and the Pentagon's highest levels.

"[Milley] stands out for his ability to relate to soldiers, but also because of his brain power," Hunter said. "You look at it and you say, 'that's the kind of thing that will appeal to someone like Secretary Carter,' someone who has spent time in management and operational positions, but also in academia."

Mackenzie Eaglen of the American Enterprise Institute, meanwhile, called the Milley pick "unconventional," a word that could also be applied to the choice of Richardson as CNO.

That selection has drawn mixed reviews, in part because Richardson would be the first director of Naval Reactors to lead the Navy, something considered a sacred trust within the service.

In a May editorial in the Wall Street Journal, John Lehman, secretary of the Navy during the Reagan administration, warned of "grave consequences" with the pick.

Eaglen noted that Richardson and Carter share a similar science background that likely helped them connect, adding that the pick is "100 percent Carter."

At the end of the day, analysts are conflicted disagree about how much impact Carter's picks really will have.

Mark Gunzinger of the Center for Strategic and Budgetary Assessments, said he believes Carter's picks the selections "all have the potential to bring fresh ideas to address tough problems. Frankly, DoD needs leaders of their caliber who are willing to challenge orthodoxy, and advocate for new operational concepts and capabilities that will shape the future force," he said. "Simply holding the line against additional budget and end strength cuts is a losing proposition in the long run. It's shaping up to be a great team."

Eaglen, meanwhile, expects more impact from the selection of Joint Chiefs than the secretaries, due to the same political realities that have Carter on a clock.

The importance of these selections "absolutely applies to the Chiefs, since they will assuredly outlast Carter, but the service secretaries will get the boot ASAP after a new administration takes over — even if it's Democratic," she said. "These are highly coveted and sought-after prestigious positions even for people who don't 'know' defense. Campaigns love to staff these with big donors and other high-profile friends."

However, Gordon Adams, an American University professor who oversaw national defense budgeting for the Clinton administration, believes the institutional nature of the chiefs will ultimately prevent any serious changes to the Pentagon.

"Overall, I don't think a secretary coming in with less than two years left of an administration can change very much," he wrote in an email. "In this case, Carter does have the chance to bring new leadership to the top of the services, but I think his chance of having them make much of a difference is minimal.

"Service chiefs have a way of riding out secretaries if they don't want to make changes," Adams added. "The services are permanent; the secretaries are temporary. So I do not expect big reforms from these appointments."

Future Appointments

One upcoming hole for Carter to fill is secretary of the Army, as John McHugh, who has served in that role since 2009, is set to step down by Nov. 1.

The leading candidate for that job, sources tell Defense News, is Eric Fanning, currently Carter's chief of staff. He previously has served in top roles with both the Navy and Air Force, including a stint as acting Air Force secretary.

Of note is Fanning's ties to Air Force Secretary Deborah Lee James. The two worked together prior to their time in the Air Force, and their relationship while serving together was seen as warm. , and Having the secretaries of the Air Force and Army seeing eye to eye could help bridge some of the gaps between the services.

One such public divide between the services has been the Air Force's desire to retire the A-10 Warthog close air support plane. During his time in the Air Force, Fanning supported the retirement of the jet, which some in Army leadership have opposed. As the Army's top civilian he could give the Air Force cover on the issue, which has been a hot button one in Congress.

No final decision has been made, and there is still a chance Fanning will not be the final choice. If he does not fill this spot, however, expect him to be on the shortlist for other top jobs as they become available under Carter.

Another potential nominee to keep an eye on is Army Maj. Gen. Ron Lewis. Lewis served less than seven months as the head of Army public affairs before Carter tapped him as his top military adviser — a role Lewis had previously filled for Carter, first at AT&L and again when Carter became deputy.

Given the close working relationship between the two men, Lewis is a name to follow for Pentagon watchers. He Lewis also fits nicely along the lines of Milley and Richardson, Hunter said.

"It probably wouldn't be a stretch to suggest Ron Lewis is destined for great things," Hunter said. "I think he's someone who has that combination of brainpower and ability to do very complex operational jobs."

Joe Gould in Washington contributed to this report.

Twitter: @AaronMehta