Speculation is swirling in Washington over the future of 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.

Mattis's relationship with President Trump is back in the spotlight after reporting in Bob Woodward's recent book detailed his frustrations with the commander in chief.

Others reports have said Mattis may leave after the midterms, with some claiming Trump has been considering a successor to the Pentagon chief for months.

Politico reported this week that Trump refers to Mattis as “Moderate Dog,” a mocking reference to the retired Marine general’s nickname, “Mad Dog."

But even before Woodward’s book, many of Mattis and Trump's disagreements on important issues have been public.

Here are five issues on which Trump and Mattis have had opposing views.

The use of torture

Trump on the campaign trail advocated for using controversial interrogation techniques that are currently banned, such as waterboarding suspected terrorists.

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He pledged to “bring back a hell of a lot worse than waterboarding,” arguing that militants plotting attacks on civilians “deserve it anyway.”

Shortly after the 2016 election, however, when Trump was mulling potential candidates to helm the Pentagon, Mattis told the president-elect that beer and cigarettes were more useful alternatives in terror suspect interrogations.

In an interview with The New York Times in November 2016, Trump said he was surprised when Mattis said he “never found it to be useful” to waterboard suspects.

Trump said Mattis told him, “‘I've always found, give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture.’”

“I'm not saying it changed my mind. Look, we have people that are chopping off heads and drowning people in steel cages, and we're not allowed to waterboard. But I'll tell you what, I was impressed by that answer,” Trump added, referring to acts committed by the Islamic State in Iraq and Syria (ISIS).

Despite that conversation with Mattis, Trump months later still said he “absolutely” thinks waterboarding works and would consider reinstating it as an interrogation technique if senior officials think it’s necessary.

Mattis, meanwhile, has been explicit that he does not condone torture, arguing that softer methods often get better results. He maintained in his Senate confirmation hearing that he is opposed to torture, and reiterated the message following Trump’s comments.

“Secretary Mattis said in his confirmation process that he will abide by and is committed to upholding international law, the Law of Armed Conflict, Geneva Conventions and U.S. law, and that has not changed,” Pentagon spokesman Navy Capt. Jeff Davis said.

The Iran nuclear deal

Trump’s decision in May to pull the United States out of the Iranian nuclear agreement caused a major rift with European allies and raised tensions in the Middle East.

Trump announced he was withdrawing from the 2015 accord between Iran and the United States, France, the United Kingdom, Germany, Russia and China that places limits on Tehran’s nuclear program in exchange for sanctions relief.

Mattis favored staying in the deal, known as the Joint Comprehensive Plan of Action. In public he had praised parts of the deal, saying that it has allowed for “pretty robust” oversight of Iran.

“I've read it now three times ... and I will say that it is written almost with an assumption that Iran would try to cheat,” he told Senate Armed Services Committee lawmakers in April, weeks before Trump’s decision.

The former Marine Corps general has said that the deal isn’t perfect, but that staying in it would be in America's national security interest.

“Absent indications to the contrary, [the Iran deal] is something that the president should consider staying with,” Mattis told the same committee in October 2017.

Withdrawing from Afghanistan and Syria

Trump on the campaign trail memorably promised to end “nation-building” missions such as efforts to train Afghan troops and stabilize the Kabul government.

After being persuaded by his Defense chief and then-national security adviser H.R. McMaster, however, Trump changed his views on withdrawing from Afghanistan. Instead, he introduced a new strategy in August 2017 that includes an indefinite time commitment and sending thousands more troops to the country.

Trump in that speech acknowledged his “original instinct was to pull out,” but he said the calculation is different “when you sit behind the desk in the Oval Office.”

More recently, when Trump reportedly sought to fire the top U.S. commander in Afghanistan, Gen. John Nicholson, because he worried the United States was “losing” the war, Mattis defended the general.

“I will tell you right now, he is our commander in the field, he has the confidence of NATO, he has the confidence of Afghanistan, he has the confidence of the United States,” Mattis told reporters.

Mattis also reportedly pulled Trump back from surprising comments he made in late March that the U.S. would "be coming out of Syria," where the U.S. is fighting ISIS militants, "like, very soon."

A week later, Trump reiterated, “I want to get out. I want to bring our troops back home.”

But Mattis and other military advisers have warned that ISIS and other terrorist groups could surge back into Syria if the United States leaves the country.

North Korea

At the height of Trump’s fiery back and forth with North Korean leader Kim Jong Un in 2017, the president appeared to rule out a diplomatic solution to halting North Korean’s missile tests.

Trump in August 2017 tweeted that the United States “has been talking to North Korea, and paying them extortion money, for 25 years. Talking is not the answer!”

When Mattis was asked about Trump’s tweet later that day he replied, “We’re never out of diplomatic solutions.”

At the June summit this year between Trump and Kim — which Mattis did not attend — Trump reportedly shocked Pentagon officials as well as allied countries when he agreed to temporarily halt several military drills on the Korean Peninsula in exchange for continued talks on a denuclearized North Korea.

The Pentagon has asserted the drills are essential to military readiness.

Mattis has since insisted that no further large-scale drills with South Korea have been put on hold as of yet.

Trump, however, said there was “no reason” for joint U.S.-South Korea military exercises at the moment.

NATO

Trump has made no secret that he has little respect for NATO, complaining on the campaign trail that the alliance was “obsolete.”

“I think NATO is obsolete," Trump told ABC News in March 2016. “NATO was done at a time you had the Soviet Union, which was obviously larger — much larger than Russia is today.”

The president has long demanded that NATO countries pay more for defense, rattling the NATO summit in Brussels in July when he demanded that all countries “immediately” hit a goal to spend 2 percent of their gross domestic product on defense spending. Such a milestone was agreed to by NATO nations at a 2014 summit, but isn’t meant to be reached until 2022.

Trump also raised the possibility that the U.S. would withdraw from the transatlantic alliance it helped create if other members kept failing to meet defense spending targets.

Trump has also routinely failed to reaffirm America’s commitment to the military alliance at important times, including at an opening ceremony for NATO’s new Brussels headquarters in May.

But Mattis, a staunch supporter of NATO, said he defended the alliance in his earliest talks with Trump.

“In my initial job interview with the president, he brought up his questions about NATO. And my response was — that I thought that if we didn't have NATO that he would want to create it because it's a defense of our values, it's a defense of democracy,” Mattis said in May 2017 on CBS News's "Face the Nation."