Acting defense secretary Mark Esper arrives at the Pentagon in Washington on June 24. (Andrew Harnik/AP)

The Pentagon is set to be led in coming weeks by its third acting defense secretary this year, according to a new succession plan made public Tuesday, underscoring the leadership upheaval at the top of the U.S. military.

Mark Esper, who became acting defense secretary on June 24 after his predecessor, then-acting defense secretary Patrick Shanahan, abruptly stepped aside, is expected to hand over to a third acting secretary, Navy Secretary Richard V. Spencer, when the White House formally submits Esper’s nomination to the Senate for confirmation.

Under a federal law known as the Vacancies Act, Esper, who has been serving as Army secretary since 2017, is required to step aside while the Senate considers his nomination for the top Pentagon job. When he will do so is not clear, as the requirement will be activated only when the White House officially transmits his nomination to the Senate.

Pentagon chief of staff Eric Chewning, speaking to reporters at the Pentagon, said the White House was expected to convey Esper’s nomination “shortly.” Pentagon officials do not know how long it would take to hold a hearing and confirmation vote after that, Chewning said, but noted that several defense secretaries had been confirmed in less than a week.

“We will not presume confirmation,” Chewning said. “It is the prerogative of the Senate to take as long as they think is necessary to examine and confirm the nominee.”

Senate Majority Leader Mitch McConnell (R-Ky.) said he was optimistic about getting a confirmed defense secretary in place before lawmakers leave for their August vacation. “We really need a confirmed secretary of defense at the earliest possible time,” he told reporters.

The rules surrounding Esper’s nomination have helped unleash a dizzying game of musical chairs at the Pentagon and intensified uncertainty in the department’s most senior ranks. This week, the admiral confirmed to become the Navy’s top officer stepped down because of his association with an officer accused of inappropriate treatment of female personnel.

Many senior Pentagon positions are filled by officials serving in a temporary capacity.

Chewning said that Esper, once officially nominated, will resume his duties as Army secretary while his nomination is considered. Ryan McCarthy, Esper’s deputy at the Army who has been serving as interim Army secretary since June 24, will move back to being the service’s No. 2.

If Esper is confirmed, the Senate could then confirm McCarthy to be Army secretary. If he is not, it is not clear what would occur because Trump has already said he will nominate McCarthy for the Army secretary job.

At the Navy, Spencer will be replaced temporarily by his deputy, Thomas Modly.

The leadership shuffle comes as President Trump continues to challenge norms that have governed civilian-military relations for decades and threatens to politicize an institution designed to be insulated from partisan fray. In particular, the department’s support for Trump’s plan to divert military funds to build his border wall has generated friction with Congress.

Chewning said that Spencer, a former military pilot who worked on Wall Street, had been receiving briefings related to responsibilities he will shoulder as defense secretary, just as Esper did last month after Shanahan pulled himself out of the running for defense secretary.

Shanahan moved up from the Pentagon’s deputy secretary post in January after the Trump administration’s only confirmed defense secretary, Jim Mattis, resigned over differences with the president. Shanahan stepped aside just weeks before he was expected to appear before the Senate for a confirmation hearing when The Washington Post and other media organizations published accounts of strife within his family.

Shanahan was never formally nominated by Trump. If he had been, he would not have been obligated to step aside during his confirmation process because the Vacancies Act carves out an exception for the deputy defense secretary.

Donna Cassata contributed to this report.

Correction: An earlier version of this story incorrectly referred to Ryan McCarthy as acting Army secretary. He has been serving as interim Army secretary since last month.