Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE's decision to quit the Trump administration is the latest indication of a Cabinet constantly being shaken up.

Mattis, who President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE announced Sunday will leave office at the end of this year — ahead of the secretary's preferred exit — is just the latest person with a high-level national security or foreign policy position to be headed out of the president's orbit.

Some have resigned, others have been ousted, and a few have moved to other posts within the administration.

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It will leave Trump with a different team in 2019.

Here's a look at the top national security-related posts that have seen turnover under Trump.

National security adviser

Three people have served as Trump’s principal adviser on national security and foreign policy issues in the White House.

The president tapped Michael Flynn, a retired three-star Army general turned vociferous campaign surrogate, to serve as his national security adviser shortly after the 2016 election.

Flynn’s tenure was extremely brief. He was forced to resign less than a month into the post over revelations that he misled Vice President Pence about contacts with the Russian ambassador during the transition. Since then Flynn has pleaded guilty to lying to the FBI about those contacts, and he cooperated with special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation into Russian interference.

Trump then appointed H.R. McMaster, an Army lieutenant general. McMaster was widely viewed as one of the more moderate voices in the administration and was said to have clashed with Trump on various issues, including the 2015 Iran nuclear deal that Trump consistently disparaged on the campaign trail.

But McMaster didn’t last long, either. In April of this year, Trump moved to replace him with John Bolton, a George W. Bush-era official known for his hawkish views on China and Iran.

FBI director

Trump’s decision to fire FBI Director James Comey James Brien ComeySteele Dossier sub-source was subject of FBI counterintelligence probe Judge will not dismiss McCabe's case against DOJ Democrats fear Russia interference could spoil bid to retake Senate MORE in May 2017 is viewed as one of the most controversial moments of his presidency.

The move came just months after Comey publicly confirmed the existence of the FBI’s investigation into whether associates of the Trump campaign coordinated with Moscow to interfere in the 2016 election.

And while the firing was predicated on a recommendation from the Justice Department that Comey be removed for his handling of the investigation into former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE’s private email server, Trump later told NBC News that the “Russia thing” factored into his decision. Mueller is said to be reviewing Comey’s firing in his probe into whether the president obstructed justice.

Comey, who has since become a frequent critic of Trump, later testified before the Senate that the president had asked him to abandon the FBI’s investigation into Flynn.

The Senate later confirmed Christopher Wray, another veteran of the George W. Bush administration, as FBI director.

Attorney general

Former Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsRoy Moore sues Alabama over COVID-19 restrictions GOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs MORE joined the administration as a trusted confidant with whom Trump had built a strong rapport during the campaign.

However, the relationship between Sessions, a former GOP senator from Alabama, and Trump turned sour after the attorney general recused himself from the Russia investigation as a result of his own communications during the campaign with Sergey Kislyak, who at the time was Russian ambassador to the U.S.

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Sessions aggressively worked to deliver on Trump’s promise to crack down on illegal immigration and criminal gangs. Still, Trump castigated him publicly — often on Twitter — for several months, telling Hill.TV in September, “I don’t have an attorney general.”

Sessions resigned at Trump’s request the day after the November midterm elections. Trump tapped Matthew Whitaker, Sessions’s former chief of staff at the Justice Department, as acting attorney general. He then nominated William Barr, formerly George H.W. Bush’s attorney general, to serve as the nation’s top cop. Both men have expressed critical views of Mueller’s Russia probe.

Homeland Security secretary

John Kelly John Francis KellyMORE, a retired Marine, was first brought on board the Trump administration as head of the Department of Homeland Security (DHS). His performance quickly captured the respect and attention of Trump, earning him a position in the White House by summer 2017.

Trump’s decision to replace Reince Priebus Reinhold (Reince) Richard PriebusLeaked audio shows Trump touted low Black voter turnout in 2016: report Meadows joins White House facing reelection challenges Trump names Mark Meadows as new chief of staff MORE with Kelly as White House chief of staff is just one example of what has become routine practice by the president — filling vacancies with officials who are already serving in high-level roles.

As such, the move left a void atop an agency central to implementing some of the president’s most controversial policies, including the travel ban and other border security measures.

Trump ultimately chose Kirstjen Nielsen Kirstjen Michele NielsenMore than million in DHS contracts awarded to firm of acting secretary's wife: report DHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections MORE, Kelly’s former deputy who also worked in the George W. Bush administration, to lead DHS. But as recently as this fall, speculation has swirled that Trump would look to remove her after the November midterms.

Secretary of State

Trump was known to have clashed with Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE, a former ExxonMobil CEO and his first secretary of State, on a variety of fronts ranging from Iran to the Paris climate accord. Tillerson reportedly called Trump a “moron” during a meeting at the Pentagon last year.

So it didn’t come as a complete surprise when Trump fired Tillerson by tweet in March, citing their disagreement on withdrawing from the Iran nuclear deal.

Trump immediately tapped then-CIA Director Mike Pompeo Michael (Mike) Richard PompeoPutin nominated for Nobel Peace Prize The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Pompeo accused of stumping for Trump ahead of election MORE to be his chief diplomat, elevating a former Kansas congressman known for his hawkish foreign policy views to a more public-facing role in the administration. Pompeo has since been a driving force in Trump’s effort to bring North Korea to the table on halting its nuclear weapons program.

CIA director

Pompeo’s transition paved the way for the first woman to take the helm of the CIA. Gina Haspel, a career veteran of the agency, was nominated by Trump and confirmed by the Senate in May, in a 54-45 vote, to lead the CIA.

Her confirmation did not come without a fight. Haspel endured broad scrutiny of her involvement in the CIA’s controversial interrogation of terrorism suspects after the 9/11 attacks.

Haspel, like other CIA directors, has kept a decidedly low profile since assuming the role, though she was dragged back into the spotlight recently as a result of press reports on the CIA’s findings about the murder of U.S.-based journalist Jamal Khashoggi at the Saudi Consulate in Istanbul.

U.N. ambassador

Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE abruptly announced in October that she would resign as U.S. ambassador to the United Nations at the end of the year.

From the start, South Carolina’s former governor had been one of the most prominent faces of the administration’s foreign policy and a fierce advocate for Trump’s moves on the international stage. Haley earned Trump’s respect early on and managed to maintain an independent streak, at times showing she was willing to disagree with the president.

Broadly, the announcement of her exit was met with disappointment among Republican lawmakers and conservatives.

Haley batted away speculation that she was leaving to challenge Trump in 2020, saying she wanted to “take a break” from public office.

Trump has since tapped Heather Nauert, the State Department spokeswoman and former Fox News anchor, to replace her, but plans to downgrade the position to below Cabinet level.

Defense secretary

Mattis sent shockwaves through Washington last week when he announced he would resign in February, citing policy disagreements with the president. The decision came days after Trump revealed he would withdraw U.S. troops from Syria, a move that earned him broad criticism.

Many had viewed Mattis as a steadying force in an administration increasingly embroiled in chaos and a fierce defender of post-World War II alliances. Democrats and Republicans reacted to the news with dismay, and fractures have since emerged between Trump and even his staunchest defenders on Capitol Hill.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellTrump 'no longer angry' at Romney because of Supreme Court stance On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline The Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power MORE (R-Ky.) said Thursday he was “distressed” to learn Mattis was resigning due to differences with the president on “key aspects of America’s global leadership.”

Trump, reportedly irked at the criticism and Mattis's own resignation letter, which laid bare the two men's differences, on Sunday announced Mattis would be out by the end of the year,

On Twitter, he said that Mattis didn’t recognize the United States is “substantially subsidizing” the militaries of “many” wealthy countries at the expense of American taxpayers. Trump also tapped Deputy Secretary of Defense Patrick Shanahan as acting Pentagon chief, ousting Mattis two months early.