Here's who Trump may pick to lead the Pentagon's nearly 3 million military and civilian personnel

Now that Donald Trump is set to become the 45th President of the United States, everyone is wondering what his potential cabinet will look like.

Perhaps most consequential is who he picks for Secretary of Defense - a civilian leadership position at the Pentagon in charge of roughly 3 million military and civilian personnel.

Here's who it could be, according to reporting from Politico and The Hill:

Lt. Gen. Mike Flynn (Ret.)

A reliable Trump surrogate on the campaign trail, Flynn is seen as a likely choice for the top spot at the Pentagon. He previously served as the head of the Defense Intelligence Agency, retiring in 2014 after 33 years in uniform. Flynn was a career military intelligence officer who served during the Cold War, Operations Desert Storm, Enduring Freedom, and Iraqi Freedom, and others.

"He's about leading from the front. He's about taking the hard jobs. He's about driving change," Adm. Michael Rogers, the head of the National Security Agency, said of him in 2014. "He's always about the men and women around him."

There's just one problem for Flynn, however. Since he's only been out of uniform for two years, he'd require a waiver from Congress to serve as Defense Secretary, since the law requires a seven year gap for military officers who want to serve as the Pentagon's civilian leader. He could still serve in some other spot, such as national security advisor.

Former Secretary of State and retired four-star Gen. Colin Powell is not a fan, however. In leaked personal emails reported by BuzzFeed News, Powell described Flynn as "abusive with staff, didn't listen, worked against policy" and called him "right-wing nutty."

Jeff Sessions (R-Ala.)

Another name being floated is Jeff Sessions, a Republican senator who has been in office since 1996. He supported the 2003 Iraq War and opposed the repeal of Don't Ask, Don't Tell and the lifting of the ban on women serving in combat roles, foreshadowing major policy reversals he could potentially implement as Defense Secretary.

Sessions has personal military experience, having served as a Captain in the US Army Reserve for 13 years. He currently sits on the Senate Armed Services Committee and has been advising Trump on national security since March. "He would obviously be a very strong fit" for Defense Secretary, said Joe Kasper, the chief of staff for Rep. Duncan Hunter (R-Calif.).

Stephen Hadley

President George W. Bush's former national security advisor may reprise that role in a Trump administration, or be tapped to lead the Pentagon as Defense Secretary.

Right now he chairs the United States Institute of Peace, a federally-funded think tank that promotes conflict resolution around the world. He's also a principal of RiceHadleyGates LLC, a consulting firm he founded with former national security advisor Condoleeza Rice, former Defense Secretary Robert Gates, and Anja Manuel, a former State Department official.

Hadley is a controversial figure. The false allegation that Iraq tried to buy uranium from Niger prior to the invasion made its way into President Bush's State of the Union speech in 2003, which Hadley later apologized for.

He also sits on the board of defense contractor Raytheon, a potential conflict-of-interest he'd have to remedy should he be tapped by Trump.

He's been hawkish on Iraq and Iran. He's also been skeptical of Russian military moves and was critical of the Obama administration's "Russian reset." He has also acknowledged the national security implications of climate change.

Sen. Jim Talent (R-Mo.)

Last but not least is former Sen. Jim Talent. Talent served in the Senate for much of the Bush administration, finally losing to Sen. Claire McCaskill in 2006. He currently serves on the US-China Economic and Security Review Commission, a group created by Congress that examines the US-China relationship and prepares an annual report on its national security implications.

Like Hadley, Talent is also an Iraq War hawk. Though he wasn't in Congress for the 2002 vote to go to war, he said in 2006 that he still would have invaded Iraq even with the knowledge there were no weapons of mass destruction.

He wants to enlarge the size of the Army, and opposes the release of detainees from Guantanamo Bay, Cuba. He has been critical of Trump's approach to NATO - setting conditions to automatic defense of NATO countries - writing that such a move could isolate America from its allies.

What they face

Whoever gets picked, the next Defense Secretary will face myriad challenges, from the ongoing fight against ISIS and China's moves in the South China Sea to the ongoing stress on the military imposed by sequestration.

A number of defense secretaries who served under Obama have criticized him for "micromanagement." Trump, it appears, seems to be more of a delegator who will let the Pentagon chief take the reins of the military.

"He will empower his SecDef to lead the way," Kasper said.

The next Defense Secretary may also end up dealing with a nuclear-armed North Korea, and Russia is very likely to test the limits of the next President in eastern Europe. He or she also needs to reinvigorate a military plagued by low morale.

Trump will also make appointments for many other positions in the Pentagon and the military services, such as service secretaries, policy undersecretaries, and advisors. Those spots may be filled from his list of retired military officers or outsiders. The current leadership at the Pentagon is already preparing for that transition.