PATRICK SHANAHAN’S decision to withdraw from consideration as defense secretary plunges the Pentagon into uncertainty just as tensions with Iran are spiking. It also reveals another case of bad judgment by President Trump, who chose to install as chief of the nation’s armed forces a man who had no record of military or government service but, it turns out, was trailed by a thick file of family turmoil.

Mr. Shanahan was a longtime Boeing executive when he was nominated by Mr. Trump as deputy defense secretary in 2017. His management experience arguably made him a reasonable choice for the No. 2 job under veteran military commander Jim Mattis. But he had zero foreign policy experience and had never served in the military. So when Mr. Mattis resigned last year because of disagreements with Mr. Trump, Mr. Shanahan was not an obvious choice to replace him.

Yet Mr. Trump quickly installed Mr. Shanahan as acting secretary, seemingly because of his readiness to go along with the president’s ever-shifting strategies toward Syria, North Korea and other strategic matters. He lacked Mr. Mattis’s gravitas and, apparently, his backbone. In the past six months, Mr. Shanahan took a back seat to Secretary of State Mike Pompeo and national security adviser John Bolton, even when they pushed for actions against Iran that risked war, such as the attempt to shut down its oil and metal exports. Yet — or maybe as a result — in May, the White House announced that Mr. Trump was nominating Mr. Shanahan to take over the defense job permanently.

On Tuesday, it emerged that Mr. Shanahan’s background investigation had bogged down. In interviews with The Post, Mr. Shanahan denied charges that he had assaulted his wife but conceded that he had prepared a defense of his son after he assaulted his mother with a baseball bat, leaving her bloody and unconscious. Mr. Shanahan professed to be distressed that what he described as a “family situation from long ago is being dredged up.” But it’s astonishing that either he or the White House would have supposed that such a record, which dates back less than a decade, would not be reexamined in the case of his nomination to head the Pentagon.

Mr. Trump said he was replacing Mr. Shanahan as acting secretary with Army Secretary Mark Esper, another former defense industry executive who does, at least, have a record of service in the military and government. But the costs of the disarray Mr. Trump has cultivated in his administration are growing. Two other key positions, the secretary of homeland security and White House chief of staff, are filled by acting appointees, as are dozens of other senior positions in the bureaucracy. The chances they will be filled by competent staff shrink as potential nominees perceive that the chief qualification for serving under this president is not relevant experience or good judgment, but unquestioning loyalty.

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