Secretary of Defense Jim Mattis reportedly said he would find it difficult to work with newly appointed national security advisor John Bolton, a noted foreign policy hawk.
Mattis was considered somewhat hawkish himself as a general in the Marine Corps, especially on Iran, but has since showed sings of believing more in the diplomatic process.
White House Chief of Staff John Kelly is also reportedly unenthusiastic about Bolton's appointment.
Secretary of Defense Jim Mattis is reportedly not happy about the new appointment of noted foreign policy hawk John Bolton as President Donald Trump's national security advisor.
Mattis "told colleagues before the appointment was announced that he would find it difficult to work with Mr. Bolton," according to the New York Times.
The reason, the Times reports, has to do with Bolton's aggressive rhetoric when it comes to the US's adversaries, especially Iran and North Korea.
As a general in the Marine Corps, Mattis himself was aggressive towards Iran — so much so that former President Barack Obama replaced him as CENTCOM commander. His selection as Trump's secretary of defense led some to worry that he would bring that attitude to the White House.
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Secretary of Defense James Mattis
Secretary of Defense James Mattis
Retired Marine Corps Gen. James Mattis walks out after a meeting with President-elect Donald Trump at the Trump National Golf Club Bedminster clubhouse at Trump National Golf Club Bedminster in Bedminster Township, N.J. on Saturday, Nov. 19, 2016.
(Photo by Jabin Botsford/The Washington Post via Getty Images)
U.S. President-elect Donald Trump and Vice President-elect Mike Pence greet retired Marine General James Mattis for a meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
Retired Marine General James Mattis departs as U.S. President-elect Donald Trump walks back into the main clubhouse following their meeting at Trump National Golf Club in Bedminster following their meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
U.S. President-elect Donald Trump stands with retired Marine Gen. James Mattis following their meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
Retired Marine Corps Gen. James 'Jim' Mattis and Operation Gratitude Founder Carolyn Blashek speak during the DIRECTV and Operation Gratitude day of service at the fifth annual DIRECTV Dealer Revolution Conference at Caesars Palace on July 23, 2015 in Las Vegas, Nevada.
(Photo by Bryan Steffy/Getty Images for DIRECTV)
Egyptian Chief of Staff of the Armed Forces Sami Anan shakes hands with US Commander of the Central Command James Mattis during a meeting in Cairo on March 29, 2011.
(KHALED DESOUKI/AFP/Getty Images)
Retired Marine Corps Gen. James 'Jim' Mattis speaks during the DIRECTV and Operation Gratitude day of service at the fifth annual DIRECTV Dealer Revolution Conference at Caesars Palace on July 23, 2015 in Las Vegas, Nevada.
(Photo by Bryan Steffy/Getty Images for DIRECTV)
Retired Marine Corps Gen. James 'Jim' Mattis, former commander of the U.S. Central Command testifies before the House (Select) Intelligence Committee on 'Threats Posed by ISIL (Islamic State of Iraq and the Levant), AQ (al Qaeda), and Other Islamic Extremists' on Capitol Hill in Washington D.C., September 18, 2014. Yesterday the House approved President Obama's plan to train Syrian rebels to counter ISIL.
(Photo by Allison Shelley/Getty Images)
Marine Corps General James Mattis, commander of the US Central Command, appears before the Senate Armed Services Committee on Capitol Hill in Washington DC, March 1, 2011. Enforcing a no-fly zone over Libya would first require a military operation to destroy the north African nation's air defense systems, top US commander General James Mattis warned Tuesday. A no-fly zone would require removing 'the air defense capability first,' Mattis told a Senate hearing. 'It would be a military operation,' he added.
(CHRIS KLEPONIS/AFP/Getty Images)
U.S. Joint Forces Command Commander James Mattis speaks during the 2010 Atlantic Council awards dinner at the Ritz Carlton Hotel on April 28, 2010 in Washington, DC.
(Photo by Kris Connor/Getty Images)
Kuwait Major General James Mattis, a high ranking Marine commander who also led troops into Afghanistan, visits Living Support Area one in Kuwait near the Iraqi border where troops are poised to begin a war against Iraq if called to do so by the President of the United States.
(Photo by Rick Loomis/Los Angeles Times via Getty Images)
U.S. Secretary of Defence Jim Mattis addresses a news conference during a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir
NATO Secretary-General Jens Stoltenberg and U.S. Secretary of Defence Jim Mattis attend a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir
U.S. Defense Secretary Jim Mattis testifies to the House Armed Services Committee on "The National Defense Strategy and the Nuclear Posture Review" on Capitol Hill in Washington, U.S., February 6, 2018. REUTERS/Joshua Roberts
U.S. Secretary of Defense Jim Mattis reviews the guard of honour during a welcoming ceremony in Hanoi, Vietnam January 25, 2018. REUTERS/Kham
U.S. Secretary for Defense, Jim Mattis, sits opposite Britain's Secretary of State for Defence, Gavin Williamson, before a meeting at the Ministry of Defence (MoD) in central London, Britain November 10, 2017. REUTERS/Simon Dawson
U.S. Defense Secretary Jim Mattis waits for the arrival of Turkish Defense Minister Nurettin Canikli prior to a meeting on the sidelines of a NATO defense ministers meeting at NATO headquarters in Brussels, Belgium November 8, 2017. Reuters/Virginia Mayo/Pool
U.S. President Donald Trump (R) and Defense Secretary James Mattis participates in a briefing with senior military leaders at the White House in Washington, U.S., October 5, 2017. REUTERS/Yuri Gripas
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But since Mattis' appointment, he has seemingly reversed his course. He argued in support for the continuation of the Iran deal last October, something that Bolton has repeatedly said should be torn up.
The defense secretary joins prominent Democrats, as well as former Bush and Obama administration officials, who have reservations about Bolton's hiring.
With the appointment of Bolton, Mattis looks even more like a moderate — and if the recent shake-ups in the White House are any indication, that may put him on the wrong side of the president.
Mattis was a close ally of now-fired Secretary of State Rex Tillerson, reportedly having breakfast with him every week. He has notably been reluctant to talk about military actions in North Korea.
"This is a diplomatically led effort," Mattis told reporters asking for details on his plans regarding North Korea in early March. "So I do not want to talk about Korea at all. I'll leave it to those who are leading the effort, the State Department and the NSC."
White House Chief of Staff John Kelly is also said to be unenthusiastic about Bolton's appointment, reportedly because he is worried that Bolton will "behave like a cabinet official rather than a staff member."
John Bolton, national security advisor, speaks during a White House press briefing in Washington, D.C., U.S., on Thursday, Aug. 2, 2018. Russian efforts to interfere in upcoming U.S. midterm elections have yet to reach the intensity of the Kremlin's campaign to disrupt the 2016 presidential vote, but they're only 'a keyboard click away' from a more serious attack, Director of National Intelligenceï¿½Dan Coatsï¿½said. Photographer: Zach Gibson/Bloomberg via Getty Images
OXON HILL, MD, UNITED STATES - 2018/02/22: John Bolton, Former United States Ambassador to the United Nations, at the Conservative Political Action Conference (CPAC) sponsored by the American Conservative Union held at the Gaylord National Resort & Convention Center in Oxon Hill. (Photo by Michael Brochstein/SOPA Images/LightRocket via Getty Images)
WASHINGTON, DC - NOVEMBER 08: US Ambassador to United Nations John Bolton speaks at the National Oversight and Government Reform Committee on moving the U.S. Embassy in Israel to Jerusalem on Capitol Hill on November 8, 2017 in Washington, DC. (Photo by Tasos Katopodis/Getty Images)
Former US Ambassador to the United Nations John Bolton speaks during the American Conservative Union Conference March 6, 2014 in National Harbor, Maryland. The annual conference is a meeting of politically conservatives Americans. AFP PHOTO/Brendan SMIALOWSKI (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
NASHUA, NH - APRIL 17: Former Ambassador to the United Nations John Bolton speaks at the First in the Nation Republican Leadership Summit April 17, 2015 in Nashua, New Hampshire. The Summit brought together local and national Republicans and was attended by all the Republicans candidates as well as those eyeing a run for the nomination. (Photo by Darren McCollester/Getty Images)
LAS VEGAS, NV - MARCH 29: Former United States ambassador to the United Nations John Bolton speaks during the Republican Jewish Coalition spring leadership meeting at The Venetian Las Vegas on March 29, 2014 in Las Vegas, Nevada. The Republican Jewish Coalition began its annual meeting with potential Republican presidential candidates in attendance, along with Republican super donor Sheldon Adelson. (Photo by Ethan Miller/Getty Images)
DES MOINES, IA - JANUARY 24: Former Ambassador to the United Nations John Bolton speaks to guests at the Iowa Freedom Summit on January 24, 2015 in Des Moines, Iowa. The summit is hosting a group of potential 2016 Republican presidential candidates to discuss core conservative principles ahead of the January 2016 Iowa Caucuses. (Photo by Scott Olson/Getty Images)
WASHINGTON - DECEMBER 4: US President George W. Bush (R) and Ambassador to the UN John Bolton (L) meet in the Oval Office of the White House December 4, 2006 in Washington, DC. Bush accepted Bolton's resignation as Ambassador to the United Nations when his term is up in January 2007. (Photo by Brendan Smialowski/Getty Images)
HOUSTON, TX - MAY 03: Former U.S. ambassador to the United Nations John Bolton speaks during the 2013 NRA Annual Meeting and Exhibits at the George R. Brown Convention Center on May 3, 2013 in Houston, Texas. More than 70,000 peope are expected to attend the NRA's 3-day annual meeting that features nearly 550 exhibitors, gun trade show and a political rally. The Show runs from May 3-5. (Photo by Justin Sullivan/Getty Images)
John Bolton, former US ambassador to the United Nations (R) and Aaron Abramovitch, Director-General of Israeli Ministry of Foreign Affairs attend a panel during the eighth annual Herzliya Conference in Herzliya, 22 January 2008. The eight annual Herzliya Conference, entitled Balance of Israel's National Security, and coordinated by the Institute for Policy and Strategy at the Interdisciplinary Center Herzliya's Lauder School of Government, started yesterday and lasts for 3 days. The theme for this year's conference is 'Israel at 60: Tests of Endurance.' AFP PHOTO/JACK GUEZ (Photo credit should read JACK GUEZ/AFP/Getty Images)
NEW YORK - OCTOBER 14: U.S. Ambassador to the United Nations John Bolton addresses the Security Council after it unanimously voted in favor of the resolution for sanctions against North Korea at the United Nations headquarters October 14, 2006 in New York City. The council voted unanimously to approve the resolution which demands that North Korea destroy all of its nuclear weapons and bans the import and export of materials used for the creation of weapons of mass destruction. (Photo by Stephen Chernin/Getty Images)
(FILES) A file picture dated 10 October 2006 shows former US ambassador to the United Nations John Bolton speaking to the media after a meeting of the five permanent members of the Security Council (Britain, China, France, Russia and United States) plus Japan, where they discussed a resolution on the North Korea nuclear situation at the UN headquarters in New York. Bolton said 21 January 2008 that Israel may have to take military action to prevent its archfoe Iran from acquiring an atomic bomb. Bolton also said that further UN sanctions against the Islamic republic will be ineffective in stopping Iran's controversial nuclear programme which Israel and the US believe is aimed at developing a bomb -- a claim denied by Tehran. AFP PHOTO/Stan HONDA (Photo credit should read STAN HONDA/AFP/Getty Images)
UNITED NATIONS, UNITED NATIONS: John Bolton (C), United States Ambassador to the United Nations, speaks to the media 13 October 2006 before a Security Council meeting about Georgia, to be followed by discussions on the North Korea resolution at UN headquarters in New York. The UN Security Council on Friday was set to consider a compromise draft resolution mandating wide-ranging sanctions against North Korea over its declared nuclear test but specifically ruling out the use of force. AFP PHOTO/Stan HONDA (Photo credit should read STAN HONDA/AFP/Getty Images)
NEW YORK - NOVEMBER 13: John Bolton, U.S. ambassador to the United Nations, speaks to the media after a meeting with the five permanent members of the Security Council and Germany concerning Iran at the French Mission to the UN November 13, 2006 in New York City. Bolton received a controversial recess appointment to the post by President Bush in August 2005 and was renominated last week, but would face confirmation from a new Democrat-controlled Senate if not voted on by the current Congress' recess in January. Democrats oppose the nomination. (Photo by Michael Nagle/Getty Images)
White House National Security Advisor John Bolton, U.S. Director of National Intelligence Dan Coats, FBI Director Christopher Wray, arrive to attend a briefing on election security in the White House press briefing room at the White House in Washington, U.S., August 2, 2018. REUTERS/Carlos Barria
U.S. National Security Adviser John Bolton attends a news conference in Moscow, Russia June 27, 2018. REUTERS/Sergei Karpukhin
HELSINKI, FINLAND JULY 16, 2018: US Ambassador to Russia Jon Huntsman Jr (C) and US National Security Adviser John Bolton (R) talking as Russian President Vladimir Putin and US President Donald Trump meet at the Presidential Palace. Alexei Nikolsky/Russian Presidential Press and Information Office/TASS (Photo by Alexei Nikolsky\TASS via Getty Images)
WASHINGTON, DC - AUGUST 02: National Security Advisor John Bolton, briefs the media on election interference, at the White House, on August 2, 2018 in Washington, DC. The administration's top security officials briefed the media on election interference. (Photo by Mark Wilson/Getty Images)
National Security Advisor John Bolton (R) attends a joint press conference of the US and Russian Presidents after a meeting at the Presidential Palace in Helsinki, on July 16, 2018. - The US and Russian leaders opened an historic summit in Helsinki, with Donald Trump promising an 'extraordinary relationship' and Vladimir Putin saying it was high time to thrash out disputes around the world. (Photo by Brendan SMIALOWSKI / AFP) (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)