Washington (CNN) Defense Secretary James Mattis may have come to terms with President Donald Trump's decision to quit the Iran nuclear deal, but he suggested the administration is still grappling with how to define US policy on countering Tehran following public calls for regime change from national security adviser John Bolton and Rudy Giuliani.

Mattis sidestepped questions from lawmakers Wednesday about whether Bolton's and Giuliani's vocal support for regime change in Iran has directly influenced the current US strategy, but offered little indication that Trump and his top advisers have reached a consensus on the administration's long-term vision.

"Our problem with Iran is not with the Iranian people. It is with the regime. But I would let Mr. Bolton and Mr. Giuliani speak for themselves," Mattis said, adding that he would prefer not to comment on the specific instances in which both men have pushed for regime change in the media.

Specifically, lawmakers expressed concerns over an op-ed Bolton published in 2015 that advocated regime regime change and a speech by Giuliani, now Trump's personal lawyer, just days before the President nixed the nuclear deal, that also endorsed the idea.

"I think it's the only way to peace in the Middle East. It's more important than an Israeli-Palestinian deal," Giuliani said about regime change for Iran.

Mattis changes his tune

Mattis himself has long pushed for a more aggressive strategy toward Tehran and in 2016 called Iran "the single most enduring threat to stability and peace in the Middle East."

But unlike Bolton, Mattis consistently advised Trump to remain in the nuclear agreement, arguing -- along with former Secretary of State Rex Tillerson -- that it was in the national security interests of the US to do so "if we can confirm that Iran is living by the agreement."

During a hearing before the Senate Armed Services Committee last October, Sen. Angus King, an independent from Maine, asked Mattis: "Do you believe it is in our national security interest at the present time to remain in the (Joint Comprehensive Plan of Action)? That is a yes or no question."

Mattis replied, "Yes, senator, I do."

"I believe at this point in time absent indications to the contrary, it is something the President should consider staying with," Mattis added at the time.

However, leading up to Tuesday's announcement, Mattis -- now without the support of Tillerson or former national security adviser H.R. McMaster -- all but stopped answering questions in public related to withdrawing from the deal.

And last week he declined to say whether he was incorporating the views of the United Kingdom, France and the United Nations nuclear watchdog charged with enforcing the Iran nuclear deal into to his discussions with Trump, telling reporters that he owed the President "a certain amount of confidentiality."

While Mattis has previously sided with officials, like Tillerson, who cautioned against scrapping the agreement, a person familiar with the dynamics told CNN that he knew he was outgunned this time around given the recent overhaul atop Trump's national security team.

According to this source, Mattis did not advocate remaining in the agreement in the weeks leading up to the announcement, knowing full well that Trump would withdraw.

"This took us over a year as we dealt with it on the inter-agency ... you saw it was not a hasty decision immediately after inauguration and I can assure you that the chairman and I were given full hearing," Mattis said on Wednesday, testifying alongside Chairman of the Joint Chiefs Gen. Joseph Dunford.

"{The President} got to a point where he just did not see that this was in our best interest to continue. It had to do with all the other things that Iran is certainly doing ... so all of this had to be mixed together, and to separate out what the chairman's or my advice was on our area, it had to be leavened with foreign policy concerns out of State, intel assessments of where they're at today and where they're going," he said.

Where does Mattis stand?

Despite Trump's decision to leave the deal, Mattis said Wednesday that the US remains in a place where it can work with allies to address some of its "shortcomings and make it more compelling."

"We will continue to work alongside our allies and partners to ensure that Iran can never acquire a nuclear weapon. We'll work with others to address the range of Iran's malign influence. This administration remains committed to putting the safety, interests and well-being of our citizens first," he said.

Mattis reassured lawmakers that the decision to leave the deal was made with the full consideration of all the related agencies but he also indicated that there is daylight between what Bolton and Giuliani have publicly said about regime change and official US policy.

While subtle, the suggestion of a split between two of Trump's most influential national security advisers has done little to temper speculation that Mattis remains more isolated than ever amid a shifting power dynamic around the President that now includes the freewheeling Giuliani, in addition to Bolton.

Mattis has done his best to downplay notions that he is concerned about working with Bolton, previously saying that he looked forward to working with the former UN ambassador and he hoped the two men held "different worldviews" to avoid "group think."

And while Mattis hopes to reconstruct the dynamic he had with Tillerson by informally meeting with Bolton and newly minted Secretary of State Mike Pompeo on a regular basis, early indications are it is unlikely that they will defer to the defense secretary in the same way Tillerson once did.

One source close to the White House told CNN that going forward, any potential conflict is likely to stem from the fact that the power dynamic is no longer heavily weighted in Mattis' favor, and situations will likely arise in which he will be forced to "share the porch" with other strong personalities whose views may conflict with his own.

Those differences manifested themselves this week when Trump announced -- despite past advice from Mattis -- that the US is leaving the Iran deal.

In Tuesday's speech -- which one senior White House official told CNN was heavily influenced by Bolton -- Trump said he would initiate new sanctions on the regime, crippling the touchstone agreement negotiated by his predecessor. Trump said any country that helps Iran obtain nuclear weapons would also be "strongly sanctioned."

Bolton's hard-edged, hawkish reputation on Iran, coupled with his rising profile within the administration, has prompted some to wonder whether he will support Trump on extreme policy initiatives even if they are opposed by Mattis.

On Wednesday, Mattis was asked about another op-ed published by Bolton prior to joining the administration titled "To stop Iran's bomb, bomb Iran," a proposal for using military force to prevent Iran from acquiring a nuclear weapon.

"Following the experience in Iraq, can the Department of Defense be confident that such an action would be limited to airstrikes and avoid another ground war involving US troops in the Middle East?" asked Democratic Sen. Tom Udall of New Mexico.

"I've had numerous conversations in the last two or three weeks with Mr. Bolton. This issue as categorized in this article has not even arisen. There is, as we all recognize, a sobering aspect of being in the positions he and I occupy," Mattis responded.