Donald Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE's nomination of James Mattis as Defense secretary is sparking questions about how well the retired Marine general will get along with Mike Flynn, the three-star general set to be national security adviser.

Trump has a history of setting up rivalries among his advisers, and it appears he’s likely to have a big one on his national security team.

Flynn reportedly sought to block Mattis's nomination weeks after Trump's win in the election. And according to The Washington Post, a power struggle has already begun between the Trump transition team and Mattis over who should serve in top slots at the Pentagon.

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The potential rivalry is almost certain to be raised at Mattis's confirmation hearings next week, especially since it gives Democrats an opportunity to express their misgivings about Flynn’s role in the White House.

Experts say that while Mattis and Flynn have things in common, there will inevitably be points of friction.

Both men are skeptical of Iran and worried about the danger posed by the Islamic State in Iraq and Syria, said Retired Army Lt. Gen. Dave Barno, senior fellow at the Atlantic Council. But the similarities may end there.

"Flynn by all indications believes that Islamic extremism is an existential threat to the future of the United States. I don't think Mattis views it that way at all," Barno said.

And whereas Flynn has criticized NATO allies for not “[paying] their bills,” Mattis sees alliances as one of the U.S.’s greatest strategic assets.

But perhaps the biggest point of debate between Flynn and Mattis will be President-elect Donald Trump's willingness to work more closely with Russia.

Mattis will head a department whose top military leaders — including Chairman of the Joint Chiefs of Staff Marine Gen. Joseph Dunford — have named Russia as the No. 1 threat to the U.S.

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Mattis himself said May 2015 at The Heritage Foundation, “in the near term, I think the most dangerous might be Russia.”

Flynn, on the other hand, "doesn't seem to have interest in that," Barno said.

In the inevitable jockeying that occurs between top Cabinet officials, there’s reason to think Flynn could have the upper hand.

For one, experts note that Flynn will be in the White House and will have Trump's ear everyday. Mattis, on the other hand, will likely see the president about once a week.

In addition, Flynn is a loyalist who has been with Trump since the early days of his campaign, establishing a rapport with the incoming president.

Flynn will also be in the White House on day one of the new administration coordinating national security policy, whereas Mattis might not be confirmed for his position until February.

Another x-factor is Steve Bannon, a conservative media figure whom Trump appointed as a top White House adviser. Bannon spent seven years in the Navy as a surface warfare officer after college and is reportedly interested in having a role in shaping military and foreign policy.

Although Mattis' confirmation hearing is scheduled for Thursday, he will still need to be confirmed by the Senate, and both houses of Congress will have to exempt him from a law requiring members of the military to be out of uniform for at least seven years before serving as Defense secretary.

"There are a lot of issues that will take place in the beginning, and Mattis is not going to be involved," Barno said.

All this could be bad news for Mattis fans and lawmakers who hope he can act as a counterweight to Flynn.

Lawmakers have made clear that they hope to draw out Mattis on some potential areas of disagreement with Trump’s team at his confirmation hearing.

When asked by a reporter what he wanted to hear from Mattis, Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans signal openness to working with Biden Loeffler calls for hearing in wake of Netflix's 'Cuties' Quinnipiac poll shows Graham, Harrison tied in South Carolina Senate race MORE (R-S.C.) replied, "Russia, bad."

Sen. Jack Reed John (Jack) Francis ReedWhen 'Buy American' and common sense collide Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-R.I.), the top Democrat on the Armed Service committee, said he would press Mattis on what he expects his role to be vis-à-vis other players.

"Part of this is going to be the questions he's asked at the hearing: What does he see as his role in respect to the National Security Council, with respect to his ... relationship to the Secretary of State. These are all questions I think in fairness you have to give him a chance to answer," he said.

Kori Schake, fellow at the Hoover Institution who co-authored a book with Mattis, said the “outsized expectation” that he can be a voice of reason within the Trump administration is unrealistic.

She called the “Mattis-as-our-salvation theory” worrisome, and said officials who served on the Trump campaign will likely be the most influential in the administration. Mattis, she said, would also be focused on the structure, size, training and equipping of the U.S. military.

Richard Fontaine, president of the Center for a New American Security, said much depends on how "White House-centric” the Trump administration’s foreign policy turns out to be.

"In the Obama administration it has been very much so, but Gen. Flynn and the President-elect will have to decide how much to empower the departments and how much to centralize," he said.

One advantage Mattis might have over Flynn is that he outranks him in prior-military service. Mattis last served as a four star commander of U.S. Central Command, and Flynn last served as a three star commander of the Defense Intelligence Agency.

Barno said retired generals are particularly mindful of rank, even in civilian life.

Mattis will also have a natural ally in Retired Marine Gen. John Kelly, who is Trump's pick for Secretary of Homeland Security and has known Mattis for four decades, Barno said.

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Yet Flynn's proximity to Trump in the West Wing will mean that Mattis will have to pick his battles, Barno said.

"He won't be on every fight every day," he said. "He's going to have to pick what are the big battles he's going to run up against."

This story was updated at 7:19 p.m.