U.S. President Donald Trump on Tuesday said he is replacing U.S. Secretary of State Rex Tillerson with CIA head Mike Pompeo, and has tapped Gina Haspel to lead the CIA.

In a tweet Tuesday, Trump thanked Tillerson, and said he has nominated Haspel to replace Pompeo, but stopped short of saying what was behind the change. If confirmed, Haspel, currently deputy director of the intelligence agency, would become its first female director.

Speaking to reporters later on the White House lawn, Trump said he made the decision to replace Tillerson “by myself,” and added that Tillerson will be “much happier now.”

It represents the biggest shakeup of the Trump cabinet so far and had been expected since last October, when reports surfaced about a falling out between Trump and Tillerson, 65, who left his position as chief executive of Exxon Mobil to join the administration.

Trump publicly undercut Tillerson’s diplomatic initiatives numerous times, including on Monday when Tillerson’s comments about Russia appeared to be at odds with those of the White House.

Tillerson also appeared out of the loop last week when Trump announced he would meet with North Korean leader Kim Jong-un and become the first sitting U.S. president to do so.

Tillerson had just returned from a shortened trip to Africa hours before Trump’s announcement. 

A senior White House official said Trump asked Tillerson to step down on Friday, but did want not to announce it while Tillerson was on a trip to Africa.

The official said Trump works well with Pompeo, a former congressman from Kansas who is seen as a loyalist within the administration, and wanted him in place before the U.S. president’s planned talks with North Korean leader Kim Jong-un and trade negotiations.

A top U.S. State Department official said, however, that Trump never explained to Tillerson the reason he was fired. 

The undersecretary of state for public diplomacy, Steve Goldstein, said Tillerson “had every intention of staying” in the job because he felt he was making critical progress in national security.

Trump and Tillerson, who had no diplomatic or political experience before becoming secretary of state, have diverged on policy numerous times, including over North Korea and Russia. On Monday, Tillerson sharply criticized Russia over the poisonings in England of a former spy and his daughter, directly blaming Moscow after White House press secretary Sarah Huckabee Sanders stopped short of doing so.