Washington (CNN) US officials in Vienna pressed European allies for changes to the Iran deal Friday, as President Donald Trump wrapped a turbulent week at home that could have big implications for the international pact to constrain Tehran's nuclear program.

Trump explained his decision to summarily replace Secretary of State Rex Tillerson with CIA Director Mike Pompeo on Thursday by citing one specific area of disagreement: Iran.

"When you look at the Iran deal, I think it is terrible," Trump told reporters after the announcement of Tillerson's ouster. "I guess he thinks it was ok. I either wanted to break it or do something and he felt a little bit different."

He added: "With Mike, Mike Pompeo , we have a similar thought process. I think it is going to go well."

Iran watchers, analysts, administration officials and diplomats say they're fairly sure they know how "it is going to go." They're readying for a harder turn on Iran after months of watching Trump abide by the agreement and a related US law, but with clear resentment and impatience after his campaign promises to "rip it up."

Changes are expected by May 12 -- the next deadline for Trump to waive sanctions against Iran and the date he has set for US allies to make changes to the Joint Commission Plan of Action, as the Iran deal is formally known, to address what he sees as its flaws.

It's a step that European allies, Iran and other signatories to the deal have said they won't accept, setting up a showdown that Trump seems eager to take on. Administration officials say Trump will happily walk away from the deal if he doesn't get what he wants.

A canceled deal would thrill critics in Washington and the Middle East, but it would dismay US military leaders who say the agreement is in US national security interests, and roil relations with Europe.

Analysts add that walking away from the deal will not only undermine US credibility, but validate Iranian claims that the US can't be trusted and complicate the administration's highest foreign policy priority: getting North Korea to relinquish its nuclear program.

"I don't care"

"I think Trump's answer would be, 'I don't care,'" says James Carafano, vice president for foreign policy at the Heritage Foundation, who ran the State Department for the President's transition team. "The President's already made the decision: He gets what he wants from the Europeans or he walks away. That's done."

Ahmad Majidyar, director of the Iran Observed Project at the Middle East Institute, said that, "Tillerson's exit and the appointment of Pompeo makes it more likely that the Trump administration will scrap the Iran deal. Already, it hangs by a very thin thread and with Pompeo, it becomes more likely that Trump cancels the deal."

Friday, a team of US officials in Vienna for talks about the deal indicated the door wasn't entirely closed.

"We can work within the Iran nuclear deal to strictly enforce it, while working on all the aspects outside of the Iran nuclear program. ... the range, the totality, of the threats Iran presents," said Brian Hook, the State Department's Director of Policy Planning, who led a team from his agency and the National Security Council.

Hook said US concerns not covered by the deal include Tehran's ballistic missiles, its presence in Lebanon, backing for Hezbollah, cyberattacks and "maritime aggression," as well as the powerful Revolutionary Guard Corps that the US says is behind much of the Mideast's destabilization.

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Issues within the deal that Trump wants changed include so-called "sunset clauses," or provisions that wind down after a few years, "and a much stronger inspection regime," Hook said. He and his team also traveled to Berlin this week to meet counterparts from Europe, "to see if we can secure a supplemental agreement addressing the deficiencies of the Iran nuclear deal," he said in Vienna.

European officials stress that they share the US concerns, and say they're making progress with the US, particularly on measures to address Iran's ballistic missiles.

On most issues, one European diplomat said, "we are close. In the end, we're pretty confident we can work on a critical mass where we can cooperate with the US."

But this diplomat drew a red line at Hook's suggestion of a supplemental to the deal. "The idea is that there is no side deal, no supplemental," this diplomat said, adding that re-opening or "disregarding the JCPOA would put us in the worst place with regard to the other concerns."

"We can't rewrite this"

A second European diplomat, referring to the six countries involved in negotiations with Iran, said the group was looking at what could be done "that would not fundamentally undermine the deal."

But this diplomat also made clear that Europe will not re-open the JCPOA. "We can't rewrite this," the diplomat said.

There are no legal consequences to leaving the deal, since it's not a treaty.

"It's just that it would have massive consequences for our partners, for our credibility, for North Korea, for validating Iran's argument that we can't be trusted," said one US official.

If Trump doesn't sign the waiver on May 12, the US sanctions go back into force, putting the US in violation of the terms of the deal. "My guess is Europe would take us to the WTO because they wouldn't want to comply with sanctions," the official said, adding that sanctions "would hit Russia, China, Europe, everyone else."

That would start in May, the same month Trump plans to meet with North Korea's leader in an unprecedented summit. Any effort to squeeze Pyongyang on its nuclear program will need a unified international front, not one riven by internal dissension over other issues.

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Even so, Trump and other administration officials have been clear that they intend to walk away from the deal unless it can be altered.

"The President made clear his view that the JCPOA gave too much to Iran for too little," a senior administration official told CNN. "He also pledged that if we were unable to secure an agreement with our allies, then he would not again waive sanctions in order to stay in the Iran nuclear deal," the official said.

Military leaders, including Defense Secretary James Mattis, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, and Gen. Joseph Votel, commander US Central Command, all believe it's within US security interests to remain in the deal and have used public appearances to make that point.

Asked during March 13 testimony before the Senate Armed Services Committee whether he agreed with Dunford and Mattis that the Iran deal served the US, Votel agreed.

"From my perspective, the JCPOA addresses one of the principle threats that we deal with from Iran," he said, "so if the JCPOA goes away then we will have to have another way to deal with the nuclear weapons program, so yes I share their position."

"This disastrous deal"

But the personnel shifts that Trump is making within his national security team are likely to deepen his impulse to pull out. While Tillerson had been seen as a moderating influence, Trump appears to be courting harder line hawks.

The President has been consulting with former Ambassador to the UN John Bolton, an advocate for abandoning the deal and executing regime change in Iran. Bolton is rumored to be a candidate to be Trump's next national security adviser.

Pompeo has been no less hawkish, calling for an end to the talks and the use of 2,000 airstrikes to obliterate Iran's nuclear program as negotiations neared their completion in 2014.

Before joining the CIA, Pompeo called for "rolling back this disastrous deal," which he argued left Iran with the ability to quickly move toward nuclear weapons capability.

The first European diplomat said their side sees some advantages to Pompeo's appointment. "I think we will have a more difficult interlocutor, but someone who is more likely to deliver Trump," the diplomat said. He "will be more aligned."

But the second diplomat flagged a nagging concern in Washington and Europe. "There's an ongoing question mark ... around what exactly will be enough for the President," this diplomat said.