Just before the 1st Marine Division advanced on the Iraqi city of Nasiriyah on March 23, 2003, Maj. Gen. James Mattis pinned a star onto each collar of his assistant division commander, Col. John F. Kelly. He was now a brigadier general, and the first to be promoted on the battlefield since the Korean War.

Not far from there, another colonel in the unit named Joe Dunford was leading his regimental combat team.

By the end of the campaign, they had fought together in places like Nasiriyah, Al Kut, and eventually Baghdad. The division they were in — along with the US Army and UK armored elements — carried out one of the most aggressive, high-speed attacks in history, and 1st Marine Division's ground march was the longest in the history of the Marine Corps, for which it earned the Presidential Unit Citation.

Those three officers went on to become four-star generals. Mattis retired in 2013 as the commander of Central Command, while Kelly retired as commander of US Southern Command in 2016. Dunford became commandant of the Marine Corps, and eventually chairman of the Joint Chiefs of Staff, where he remains.

All three remain good friends. And if President-elect Donald Trump's picks for his Cabinet are all confirmed, they'll once again be serving together — only this time, it'll be in the White House.

Secretary of Defense James Mattis

AP Photo/Matt Dunham

Mattis has often been praised by senior leaders at the Pentagon as both a strategic thinker with an encyclopedic knowledge of history and an incredible leader. His legendary status among Marines mainly originated from his command of 1st Marine Division, where he popularized its motto, "No better friend, no worse enemy."

The 66-year-old retired general is the only pick that has a legal roadblock in front of him. A 1947 law, updated in 2008, requires military officers to be out of uniform for at least seven years before leading the Pentagon. Mattis would need a waiver, which Republicans have already signaled support for.

When asked recently if he was concerned by Mattis as Trump's pick, Gen. Joe Dunford just said, "No."

If confirmed, Mattis would replace Defense Secretary Ash Carter, who supports Mattis and called him "extremely capable."

Secretary of Homeland Security John Kelly

John Kelly just accepted Trump's request for him to serve as the head of the Department of Homeland Security, according to CBS News.

Like Mattis, he is a blunt speaker who opposes the closure of the detention facility at Guantanamo Bay.

"What tends to bother them is the fact that we're holding them there indefinitely without trial. ... It's not the point that it's Gitmo," he told Defense One earlier this year. "If we send them, say, to a facility in the US, we're still holding them without trial."

Kelly is also the most senior-ranking military official to lose a child in combat since 9/11. His son, Lt. Robert Kelly, was killed by an improvised explosive device in Afghanistan in 2010.

If confirmed, Kelly would replace Jeh Johnson.

Chairman of the Joint Chiefs Gen. Joe Dunford

US Marine Corps

Joe Dunford is the last of the three generals who is still in uniform. He served briefly as commandant of the Marine Corps before President Barack Obama nominated him as the chairman of the Joint Chiefs in May 2015. He earned the nickname "Fighting Joe" during his time with 1st Marine Division.

Dunford has been in the Marine Corps for 39 years, less than Mattis' 44 years and Kelly's 45. His chairmanship term is scheduled to run through 2017. Though the Joint Chiefs are not part of the president's Cabinet, they are appointed by — and serve as the top military advisers to — the president.

Trump is likely to replace many of Obama's appointees, but Dunford may not be one of them.

Typically, Joint Chiefs chairmen serve two terms, and having comrades like Mattis and Kelly in Dunford's corner would make it much harder for Trump to replace him.

Trump has floated other generals and admirals for his Cabinet, including Gen. David Petraeus for secretary of state and Adm. Michael Rogers for director of national intelligence. Michael Flynn, his controversial choice for national security adviser, is a retired lieutenant general who headed the Defense Intelligence Agency.

These choices don't come without pushback. Some, like Phillip Carter, a former Army officer with the Center for a New American Security, have argued that Trump's reliance on retired military brass for traditionally civilian-led organizations could jeopardize civil-military relations.