Acting Defense Secretary Patrick Shanahan has given up his post and withdrawn his nomination to take the job permanently, President Donald Trump tweeted, announcing yet another Cabinet change.

Army Secretary Mark Esper will be the new acting Pentagon boss, Trump said. Shanahan, a former defense industry executive, replaced retired Marine Corps Gen. James Mattis when he stepped down, citing differences with Trump.

“I know Mark, and have no doubt he will do a fantastic job!” the president wrote.

Shahahan, 56, served for more than 30 years as an executive at the Boeing Co. before he became deputy secretary of Defense in June 2017. He had said he already had recused himself from official decisions involving his former employer, but the matter was expected to be a focus of his now-scuttled confirmation hearings.

Shanahan’s sudden departure comes amid tensions with Iran over its alleged attacks on oil tankers in Middle East waterways that are key to global commerce — and the U.S. economy.

[After Iran briefings, Democrats in Congress want to know more, sooner]

Notably, the White House deployed Secretary of State Mike Pompeo to U.S. Central Command headquarters in Tampa, Fla., to visit troops that will be part of a new deployment intended to potentially counter Iran’s actions and provide a show of force to deter future aggressive actions.

The president has praised Shanahan for months, but reportedly had grown concerned about the outgoing defense secretary’s nomination stalling in the Senate. One report said the commander in chief was talking to officials during a European trip earlier this month about alternative nominees.

Trump is relying on more acting Cabinet heads than his predecessors, once saying “my actings” give him more “flexibility.” White House aides have never defined that, but lawmakers and experts say he apparently views them as required to be more loyal if they want their temporary jobs to become permanent.

On Friday, a host on the “Fox & Friends” morning show erroneously asked Trump whether he still intended to nominate Shanahan, which he did in early May.

What followed was something less than a glowing presidential endorsement.

“Well, I have, defense secretary. I have. It’s done. I put it out,” Trump told Fox News Friday morning. “Yeah, it’s done from the standpoint of the nomination. Wait, wait, wait, Pat Shanahan was nominated two weeks ago. Yeah, no, I put it out, I put it out officially. Now he has to go through the process. He’s now going through.

“He’s been recommended, now he has to be approved by Congress. We are going to see,” he said. “We are going to see, Pat Shanahan has been recommended for the job.”

On June 12, White House counselor Kellyanne Conway told MSNBC that Trump had “great confidence” in Shanahan and his decision to tap him for the job permanently.

The final straw appears to have been reports that the former nominee was involved in a violent domestic incident nine years ago with his then-wife. At the time, Shanahan and his spouse told police they punched one another. He since has said he did not, but the woman — who now goes by Kimberley Jordinson — says she stands by her version of the incident.

As he left the White House later Tuesday for a campaign rally in Florida, Trump  said he learned of Shanahan’s domestic incident on Monday. He said Shanahan “came in” Tuesday morning and explained the public disclosure means “a very hard time” is ahead for him and his family. Trump defended his team’s vetting process, saying he is unconcerned the incident was not uncovered sooner.

Democrats and other critics were expected to raise other questions, including about whether Shanahan had the proper experience to run the U.S. military.

“This is a very different job than his previous position,” Jack Reed of Rhode Island, the top Senate Armed Services Committee Democrat, said in a May statement. “Serving as Secretary of Defense is an immense task and I look forward to hearing directly from the Acting Secretary during the confirmation process.”

During his tenure as deputy secretary and acting secretary, Shanahan was active in trying to force the military services to shift focus from almost exclusively fighting terrorists to preparing more for possible conflict with Russia or China.

That has meant using his offices to emphasize more spending on new technologies such as artificial intelligence. He worked hard, too, to make a greater percentage of military aircraft battle-ready.

He also defended and pushed several Trump policies that many Democrats oppose, such as the White House’s proposed new military service known as a Space Force. And Shanahan has been a stalwart defender of Trump’s use of U.S. soldiers and defense dollars to address immigration on the southern border.

John M. Donnelly contributed to this report.