PEOPLE Explains: All About John Kelly, the New White House Chief of Staff Who Lost a Son in Combat

Retired Gen. John Kelly has officially been sworn in as White House chief of staff, replacing Reince Priebus after a week of turnover and turmoil in President Donald Trump‘s administration.

Kelly began his tenure on Monday morning, with the president praising the former Homeland Security secretary during a Cabinet meeting. Said Trump, “General Kelly, he will be chief of staff, as you know, we all know him, we respect him, admire what he’s done.”

“I just want to congratulate him on the great job he’s done with Homeland Security, and I have no doubt that he will be an absolutely superb chief of staff,” added the president.

Unlike some of the Trump administration’s other recent additions, Kelly has had a storied military career and government experience. Here’s what you need to know about the new chief of staff:

He’s a Retired 4-Star General

When Kelly, 67, retired earlier this year he was the longest-serving general in the armed forces, reports ABC News.

Kelly enlisted in the Marine Corps in 1970, deploying for the first time the following year when he was just 20. According to the outlet, Kelly continued to rise in the ranks before being named the special assistant to the supreme allied commander in Europe in 1999. Three years later, he was promoted to brigadier general, serving as the assistant division commander with the 1st Marine Division.

Eventually, he was promoted to major general, before commanding Marine Forces Reserves and Marine Forces North. He became the head of SOUTHCOM, or the DoD’s U.S. Southern Command, in 2011. SOUTHCOM includes military and civilian personnel from the Army, Navy, Air Force, Marine Corps, Coast Guard and a few other federal agencies.

He Oversaw Operations at Guantanamo Bay

As head of SOUTHCOM, the infamous Guantanamo Bay U.S. military base in Cuba fell under Kelly’s purview. Kelly oversaw the commander of operations of the Pentagon detention center.

During his tenure, Kelly was a known proponent of the controversial complex – even amid former President Barack Obama’s efforts to close it – and told the Military Times, “Every [prisoner] has real, no-kidding intelligence on them that brought them there. They were doing something negative, something bad, something violent, and they were taken from the battlefield.

“There are a lot of people that will dispute that, but I have dossiers on all of them, built and maintained by the intelligence community, both military and civilian. There are no innocent men down there.”

He’s a Gold Star Father Whose Son Died in Afghanistan

General Kelly is the highest-ranking military officer to lose a child in Iraq or Afghanistan, according to the New York Times.

Kelly’s son, Lt. Robert Michael Kelly, was killed in 2010 while leading a platoon in Afghanistan. The lieutenant, 29, stepped on a land mine and was killed instantly.

According to the Washington Post, Kelly’s son enlisted in the Marines in 2003 after graduating from Florida State University. In an email sent after he received the news of his son’s death, Kelly wrote of the moment, “It was disorienting, almost debilitating,” per the Post.

“At the same time my mind went through in detail every memory and image I had of Robert from the delivery room to the voice mail he’d left a few days before he died. . . . It was as graphic as if I was watching a video. . . It really did seem like hours but was little more than a second or so,” he wrote, per the Post.

Ironically, Kelly Was Introduced to Trump by Reince Priebus

The outgoing chief of staff is, ironically, the original link between Kelly and the president, according to the Wall Street Journal.

In an interview with NBC News’ Pete Williams at the Aspen Security Forum earlier this month, Kelly said, “I literally did not know Mr. Trump at all and I didn’t know anyone that knew Mr. Trump. About 10 days after the election, I was watching college football with my wife on a Saturday afternoon, I got a call, Reince Priebus.”

RELATED VIDEO: Watch: Natasha Stoynoff Breaks Silence, Accuses Donald Trump of Sexual Attack

In fact, Kelly said he was so surprised by the call that he thought it was a practical joke played by one of his friends. But it was Priebus, whom Kelly said told him, ” ‘Mr. Trump would like to have an opportunity to talk to you maybe about going into the administration.’ ”

He Expressed His Support for the Proposed Border Wall While Serving as the Head of Homeland Security

Following his November phone call with Priebus, Kelly met with the president and was officially nominated to head the Homeland Security Department in early December.

During Kelly’s confirmation hearings, he parted from the president on some issues – including the proposed Mexico border wall, reportedly saying it wouldn’t be built “anytime soon,” according to the Washington Post. Kelly also assured he would “keep an open mind” on Obama’s Deferred Action for Childhood Arrivals (DACA) program, despite Trump’s plan to reverse it.

Kelly’s stance changed further into his tenure, however. During an April appearance on Face the Nation he called the wall “essential.”