Donald Trump just announced that he is replacing Secretary of Defense Gen. James Mattis immediately and replacing him with Deputy Secretary of Defense Patrick Shanahan.

The abrupt decision comes as Trump’s anger has boiled over Mattis’s resignation letter, which was widely seen as a rebuke of the president’s foreign policy.

Trump tweeted, “I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!”

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!

— Donald J. Trump (@realDonaldTrump) December 23, 2018

Mattis resigned from the Trump administration on Thursday in protest of the president’s abrupt announcement to withdraw all U.S. forces from Syria.

Mattis wrote in his resignation letter that he felt he could no longer continue to execute Trump’s policies, and cited the importance of what he called America’s “unique and comprehensive system of alliances and partnerships.”

“Because you have the right to have a secretary of defense whose views are better aligned with yours … I believe it is right for me to step down from my position,” Mattis said.

In his letter, Mattis said his last day would be on Feb. 28 to ensure a smooth transition to his successor.

Shanahan, a former Boeing executive, played a key role in spearheading the recovery of the company’s 787 program.

Trump nominated him to the Pentagon’s second-highest civilian position in March 2017, and he was confirmed by the Senate in July 2017.

Twitter was shocked:

His feelings are hurt so he lashes out … he hasn't matured one iota in seven decades.

— Oaxaca Flea (@Oaxaca_F_Aucoin) December 23, 2018

And some said that a woman was too emotional to be President.

— Tgage (@tgagemurphy) December 23, 2018

72 year old man baby.

— Jenn B (@Jennicuti) December 23, 2018

He is jealous of the respect that Mattis has earned. Trump will never receive that same respect except from his cult.

— MomCavy41 (@Cavy41Mom) December 23, 2018

Pettiness and vengefulness is the signature and face of our president. There has never been a president in our history as base and evil as this one.

— Lawrence Kohler (@ltkohler) December 23, 2018

WATCH:

.@CBSDavidMartin on Mattis on @FaceTheNation: “We have a statement from the Pentagon saying the Secretary of Defense serves at the pleasure of the President. And I interpret that as meaning that the Secretary has been asked to go at once.” pic.twitter.com/K48DjKzE8m

— Face The Nation (@FaceTheNation) December 23, 2018