WASHINGTON >> After months of unexplained delays, Acting Defense Secretary Patrick Shanahan stepped down today before his formal nomination ever went to the Senate, citing a “painful” family situation that would hurt his children and reopen “wounds we have worked years to heal.”

President Donald Trump announced Shanahan’s departure in a tweet, and said that Army Secretary Mark Esper would be the new acting Pentagon chief.

“It is unfortunate that a painful and deeply personal family situation from long ago is being dredged up and painted in an incomplete and therefore misleading way in the course of this process,” Shanahan said in a statement. “I believe my continuing in the confirmation process would force my three children to relive a traumatic chapter in our family’s life and reopen wounds we have worked years to heal. Ultimately, their safety and well-being is my highest priority.”

He provided no other details.

But as a result of that situation, Shanahan said, he asked to be withdrawn from the nomination process and he resigned from his previous post as deputy defense secretary. He said he would work on an “appropriate transition” but it wasn’t clear how quickly he will leave the job.

In his tweet, Trump simply said that Shanahan had done “a wonderful job” but would step aside to “devote more time to his family.”

And, in noting Esper’s move, Trump added, “I know Mark, and have no doubt he will do a fantastic job!”

Officials were meeting today to determine transition plans.

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

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.

His in 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.