FILE - In this Feb. 14, 2019 file photo, acting Secretary of Defense Patrick Shanahan speaks about the situation in the Persian Gulf region during a meeting with Portuguese Minister of National Defense Joao Cravinho, at the Pentagon. President Donald Trump announced on June 18 that Shanahan will not move forward with the confirmation process to be Defense Secretary. (AP Photo/Manuel Balce Ceneta, File)

FILE - In this Feb. 14, 2019 file photo, acting Secretary of Defense Patrick Shanahan speaks about the situation in the Persian Gulf region during a meeting with Portuguese Minister of National Defense Joao Cravinho, at the Pentagon. President Donald Trump announced on June 18 that Shanahan will not move forward with the confirmation process to be Defense Secretary. (AP Photo/Manuel Balce Ceneta, File)

WASHINGTON (AP) — President Donald Trump announced Tuesday that Acting Secretary of Defense Patrick Shanahan is withdrawing his nomination to lead the Pentagon.

The abrupt announcement came in a tweet from the president, who said Shanahan had done “a wonderful job” but would step aside to “devote more time to his family.”

The president added that the Secretary of the Army, Mark Esper, will be the new acting secretary.

“I know Mark, and have no doubt he will do a fantastic job!” Trump said in a second tweet on the subject.

The post atop the Pentagon has not been filled permanently since Gen. James Mattis retired in January following policy differences with Trump.

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Trump announced in May that he would nominate Shanahan but the formal nomination process in the Senate had been inexplicably delayed

Shanahan, a former Boeing executive, has been leading the Pentagon as acting secretary since Jan. 1, a highly unusual arrangement for arguably the most sensitive Cabinet position.

In his tenure at the department he’s had to deal with a wide array of international hotspots, ranging from missile launches by North Korea to the sudden shift of military ships and aircraft to the Middle East to deal with potential threats from Iran.

Shanahan, 56, had extensive of experience in the defense industry but little in government. In more than four months as the acting secretary, he focused on implementing the national defense strategy that was developed during Mattis’ tenure and emphasizes a shift from the resources and tactics required to fight small wars against extremist groups to what Shanahan calls “great power” competition with China and Russia.