President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE fired back at Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE after his former secretary of State called the president undisciplined, unread and willing to break the law.

"Mike Pompeo Michael (Mike) Richard PompeoOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Overnight Defense: House Democrats unveil stopgap spending measure to GOP opposition | Bill includes .6B for new subs | Trump issues Iran sanctions after world shrugs at US action at UN Navalny calls on Russia to return clothes he was wearing when he fell ill MORE is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed. He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell. Now it is a whole new ballgame, great spirit at State!" Trump tweeted.

Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed. He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell. Now it is a whole new ballgame, great spirit at State! — Donald J. Trump (@realDonaldTrump) December 7, 2018

Trump's extraordinary attack on his former top diplomat came one day after Tillerson offered a blistering criticism of his ex-boss, one of the clearest signs of the strained relationship between the two men.

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"So often, the president would say here's what I want to do and here's how I want to do it and I would have to say to him, 'Mr. President, I understand what you want to do but you can't do it that way. It violates the law,'" Tillerson said at a fundraising event in Houston.

The former Exxon Mobil CEO said he did not meet Trump until the day he was named secretary of State and described how their relationship got off on the wrong foot almost instantly, saying they "did not have a common value system."

“I'd say, 'Here's what we can do. We can go back to Congress and get this law changed. And if that's what you want to do, there's nothing wrong with that.' I told him, 'I'm ready to go up there and fight the fight, if that's what you want to do,'” he said, adding that Trump would show frustration during those conversations.

Tensions between Trump and Tillerson reached a high point last fall when it was reported that Tillerson privately called Trump a "moron" after a contentious meeting at the Pentagon in the summer of 2017. The then-secretary of State pushed back on the report and praised the president but never specifically denied using the word.

Despite Trump's claim he "couldn't get rid of him fast enough," Tillerson led the State Department until March 2018, serving 405 days at Foggy Bottom under Trump.

Trump announced in December 2016 he would take the unconventional step of nominating Tillerson before his administration had even begun, saying in a statement that his "broad experience and deep understanding of geopolitics make him an excellent choice for secretary of State."