By Kaitlan Collins, Nicole Gaouette and Dan Merica, CNN

(CNN) -- President Donald Trump announced Tuesday that he has fired Secretary of State Rex Tillerson and will nominate CIA Director Mike Pompeo to succeed him, replacing his top diplomat ahead of a potential high-stakes sitdown between the US President and North Korean leader Kim Jong Un.

Tillerson's departure follows months of tension between him and Trump. Gina Haspel, the current CIA deputy director, stands to take over the agency, Trump tweeted Tuesday morning.

Speaking to reporters at the White House, Trump said he respected Tillerson's "intellect" and said he "got along well with Rex."

"I think Rex will be much happier now," Trump said.

Tillerson did not speak to Trump and is unaware of the reason behind his firing, Undersecretary for Public Diplomacy Steve Goldstein said.

Trump "thought it was the right time for the transition with the upcoming North Korea talks and various trade negotiations," a senior administration official said, adding that Trump asked Tillerson to step aside on Friday.

Asked how Tillerson learned of his dismissal, Trump said Tuesday that "Rex and I have been talking about this a long time." He specifically mentioned the Iran nuclear deal as an example of disagreement.

"We were not really thinking the same," Trump said. "With Mike Pompeo, we have a similar thought process."

Trump eyed Pompeo for months

Trump has wanted Pompeo as his secretary of state for months now, and the White House began planning for him to take the job last fall, sources told CNN. Trump told reporters Tuesday that he and Pompeo are "on the same wavelength" and "the relationship has always been very good and that is what I need."

Trump's anger at Tillerson after it leaked last year that his secretary of state called him "a moron" never subsided, and many in the White House saw their differences as irreconcilable.

Tillerson had few, if any, allies in the West Wing. Though chief of staff John Kelly was initially on his side when he took over, he eventually grew weary of defending him -- especially after the "moron" remark, which Kelly saw as insubordination on Tillerson's part.

Sources close to the President say it was clear Tillerson didn't support Trump. They say Tillerson wanted to handle foreign policy his own way, without the President. Trump didn't feel that Tillerson backed him, a source told CNN.

High-stakes moment

Tillerson's departure comes just as the Trump administration embarks on its most difficult and ambitious foreign policy goal to date -- engaging the nuclear armed North Korean regime. Trump is set to meet Kim by the end of May.

It was a rocky tenure for Tillerson. Since his swearing in on February 1, 2017, Tillerson had to contend with a President who publicly undercut him as well as tension with Trump son-in-law Jared Kushner, who effectively ran a shadow State Department on Middle East issues. There was also competition from US Ambassador to the UN Nikki Haley and a litany of complaints from diplomats, State Department staff and others in Washington that he was running a deeply dysfunctional agency.

Tillerson's cost cutting has lead to the agency's senior tiers "being depleted at a dizzying speed," and "a decapitation of its leadership ranks," Amb. Barbara Stephenson, president of the American Foreign Service Association (AFSA), a union for US foreign service personnel, wrote in her group's publication.

"There is simply no denying the warning signs that point to mounting threats to our institution -- and to the global leadership that depends on us," Stephenson wrote.

Tillerson aggressively pushed back against such criticisms in a November 28 appearance, portraying it as an insult to State Department staff. "I'm offended on their behalf when people say somehow we don't have a State Department that functions," Tillerson said. "I can tell you it's functioning very well from my perspective."

Tillerson on thin ice for months

But a steady drip of negative news, and reports of Tillerson's alleged resentment over Ivanka Trump leading a delegation to the Global Entrepreneurship Summit in India in November, continued to undermine Tillerson.

His ouster was preceded by a painfully public airing of his troubles with the President, heightened when one lawmaker faulted Trump for his tendency to "publicly castrate" the secretary of state. That exchange forced Tillerson, when asked about the comment by CNN's Jake Tapper, to declare, "I checked. I'm fully intact."

The lawmaker, Sen. Bob Corker, the Tennessee Republican who chairs the Senate Foreign Relations committee, told The Washington Post last fall that Trump repeatedly neutered Tillerson with tweets that undermined or flatly contradicted policies he was pursuing. That, Corker said, shut down options for the US and damaged Tillerson's efforts to peacefully resolve tensions with North Korea.

By that point, in mid-October, an administration official was telling CNN that Tillerson was on thin ice, even as the President was publicly declaring he had confidence in his top diplomat.

"It certainly isn't a good relationship and its problems that have been building," the official said of Tillerson's fate in the Trump administration. "I think everyone is trying to stick it out for a variety of selfish reasons. But not for the same reasons."

The castration episode followed an extraordinary October 4 public statement in which Tillerson stressed his commitment to his job as secretary of state, but didn't definitively deny an NBC report that he had called Trump a "moron."

That report also detailed Tillerson's "fury" about the ways Trump has undermined him publicly on several foreign policy initiatives and his thoughts about resigning.

Calling the story "erroneous" during his remarks, Tillerson pointed the finger at "some who try to sow dissension" to undermine the President's agenda and said he has been asked "repeatedly" if he's going to step down.

"For some reason, it continues to be misreported," Tillerson complained. "There's never been a consideration in my mind."

The incident cost Tillerson the support of Kelly, a top White House official told CNN.

Kelly, a retired four-star Marine general who was once Tillerson's fiercest defender in the West Wing, stopped defending him privately, fed up with the moron remark because he saw it as insubordination. As one official described it, he has grown weary of trying to defend the indefensible.

Even as Tillerson lost support in the White House, some lawmakers expressed concern about the prospect of his departure. Many saw Tillerson, along with Defense Secretary James Mattis and national security adviser H. R. McMaster, as a serious defender of US national security interests, as opposed to more ideological or inexperienced voices in the White House.

Difficult situation for Tillerson

As Tillerson exits the national stage, he's seen by some as a man who tackled a job he hadn't sought with the diligence and dedication prized by another organization he led, the Boy Scouts of America. Many point to the fact that he was in a situation that made it very difficult to succeed.

Tillerson started the job diverging with Trump on any number of issues, from trade, climate change, Russian interference in the election and Iran policy. And he encountered headwinds from the President who publicly contradicted or undermined his policies on using diplomacy to defuse tensions with North Korea, and on resolving a dispute between Gulf allies and a host of other issues.

"Trump has consistently undermined -- even humiliated -- his top diplomat" on issues that ranged from tensions among Persian Gulf allies to handling North Korea, said Aaron David Miller, a vice president at the Wilson Center, and Richard Sokolsky, a senior fellow at the Carnegie Endowment for International Peace, in an opinion piece for CNN.

Others have dismissed Tillerson's tenure, saying his business experience hadn't translated into government leadership and pointing to the downsized and demoralized State Department he leaves behind, with many senior diplomatic positions still unfilled.

Critics from both parties said his proposed cuts of up to 30% were damaging US interests, with former Secretary of State Madeleine Albright declaring in November that the state of the agency was a "national security emergency."

Foreign envoys also voiced concerns about Tillerson's State Department. In October, one Washington-based envoy described how ambassadors were finding ways to either bypass the State Department or develop work-arounds, because there were no senior officials in place to speak to or because the usual channels within State no longer worked.

"Technically, the State Department has vanished, has disappeared," the envoy said. "It's totally dysfunctional."

This story is breaking and will be updated.