Former National Security Adviser Michael Flynn's abrupt resignation made waves on Monday evening, as pressure mounted amid controversy over his communications with a Russian ambassador.
Nevertheless, as the principal adviser on national security issues, the opening in President Donald Trump's administration is a crucial one that the administration is most likely to fill quickly.
Vice Adm. Robert Harward, a former deputy commander of US Central Command, is the front-runner to replace Flynn, according to Washington Post reporter Robert Costa. The New York Times also reports that Harward is the leading candidate to take over.
Former US National Security Advisor Michael Flynn. | via Flickr
The position is appointed by the president, and does not require a lengthy confirmation hearing from the Senate.
Here are five possible candidates that may become the next national security adviser to Trump:
Peter Jacobs contributed to this report.
Retired Gen. David Petraeus
Retired Gen. David Petraeus' career includes 37 years of service in the US Army and a role as Director of the Central Intelligence Agency.
In addition to commanding the entire coalition force in Iraq, the four-star general headed US Central Command (CENTCOM), which oversees all operations in Middle East.
Petraeus was briefly considered for Secretary of State by the Trump administration.
Stephen J. Hadley
Stephen Hadley served as the National Security Adviser to President George W. Bush from 2005 to 2009.
He served on several advisory boards, including defense firm Raytheon, and RAND's Center for Middle East Public Policy. Together with former Secretary of State Condoleezza Rice and former Secretary of Defense Robert Gates, he helps head the international strategic consulting firm, RiceHadleyGates LLC.
He also wrote the "The Role and Importance of the National Security Advisor," which, as the title implies, is an in-depth study of the National Security Adviser's role.
Retired Gen. Keith Kellogg
Major General Joseph K. Kellogg Jr., USA (uncovered)
As the interim National Security Adviser filling in for Michael Flynn, retired Lt. Gen. Keith Kellogg was the chief of staff for the Trump administration's National Security Council (NSC).
Screengrab via CNN/YouTube
Tom Bossert, a cybersecurity expert, serves as the Homeland Security Adviser in the White House.
The former Deputy Homeland Security Adviser to President George W. Bush co-authored the 2007 National Strategy for Homeland Security, the government's security policies established after the 9/11 terror attacks.
In a 2015 column in The Washington Times, Bossert seemed to defend the wars in Iraq and Afghanistan by writing, "To be clear, the use of military force against Iraq and Afghanistan was and remains just ... The use of force in Iraq was just and, at the time, necessary, even if Mr. Obama disagrees with how things went."
Retired Vice Adm. Robert S. Harward
Retired Vice Adm. Robert S. Harward is a US Navy SEAL and the former Deputy Commander of US Central Command (CENTCOM).
He served as the commander of SEAL Team 3 and was the Deputy Commanding General of Joint Special Operations Command (JSOC). Harward also served on the National Security Council as the Director of Strategy and Policy for the Office of Combating Terrorism, and is also the CEO for Lockheed Martin in the United Arab Emirates.