Donald Trump tapped retired Lt. Gen. Keith Kellogg to serve as acting national security adviser, elevating the man who served as the national security council's chief of staff to the top role on a temporary basis.

Kellogg was in the running to hold the position full-time, as well as retired Gen. David Petraeus and former Vice Admiral Bob Harward, a source told CNN Monday evening. A senior official told CNN Tuesday that Harward is considered the top contender for the job.

Here's a few things to know about the three men who stand a chance to take the job:

Like Flynn, Kellogg is a three star former lieutenant general in the Army who retired and went on to serve in the Trump administration. He worked directly under Flynn serving as the council's chief of staff for less than a month before the controversy over Flynn's contact with Russian ambassador Sergey Kislyak forced Flynn out of the job and Kellogg into acting leadership of the council.

Michael Flynn

Before joining the Trump administration, Kellogg served as a foreign policy adviser to Trump during the campaign.

Kellogg spent decades in the military and rose through the ranks before retiring at the outset of the Iraq War. He continued to work with the US government and within the national security arena, taking a leading role in the Coalition Provisional Authority -- a group tasked with reorganizing the Iraqi government and military after its dissolution and reformation under President George W. Bush.

As more scrutiny draws on Kellogg, he is sure to face pushback for his roles in the private sector and the Iraq War.

Also like Flynn, Petraeus is a former military man who served for years before a major rise and fall under Barack Obama.

Petraeus was one of the key military figures to codify the US' counterinsurgency approach to conflicts in the Middle East. He became hugely popular for spearheading the military surge in Iraq to curb heightened violence in the late 2000s, according to CNN report.

At the time, the Bush administration and many throughout Washington dubbed the surge a success and Petraeus was applauded for it. Then when Gen. Stanley McChrystal was forced to resign after an earthshaking article from Michael Hastings, a now-deceased freelance journalist who was published in Rolling Stone, Obama tapped Petraeus to take over from McChrystal as the military leader in Afghanistan.

Petraeus led troops in Afghanistan before another major Obama administration switch-up resulted in Petraeus moving to lead the CIA.

In a stunning fall from grace, he resigned his position over an extramarital affair with his biographer. A Justice Department investigation accused Petraeus of sharing classified information with the woman whom he was having an affair with. He pleaded guilty and agreed to serve two years probation and pay $100,000 fine.

Trump reportedly considered him for secretary of state and the vice presidency, but ultimately offered him no role in the administration. If tapped to be national security adviser, he would still be under probation until April.

Unlike Flynn, Kellogg and Petraeus, Harward has served in the Navy.

According to the Navy, he served in both east and west coast SEAL teams and took a tour in the White House where he worked with the national security council. During the Obama administration, he went on to serve as deputy commander of US Central Command.
He grew up in a Navy family and spent some of his early years in Iran where his father served as a naval officer.

Since retiring from the military, he has served as an executive in Lockheed Martin UAE.

Incidentally, emails made public as a result of a Washington Post FOIA request over the Petraeus resignation showed correspondence between Harward and the man who is now secretary of defense, then Gen. James Mattis.