In this June 24 file photo, acting Secretary of Defense Mark Esper arrives for work at the Pentagon. (Erik S Lesser/EPA-EFE/REX/Shutterstock)

President Trump officially nominated Mark T. Esper to become the nation’s next defense secretary on Monday afternoon, prompting the retired Army officer to step down as acting defense secretary and hand over his duties to Navy Secretary Richard V. Spencer for the duration of the confirmation process. 

Esper, a former defense lobbyist, congressional staffer and Pentagon official, has returned to his previous role as Army secretary until he is confirmed. His confirmation hearing before the Senate Armed Services Committee is scheduled to begin on Tuesday morning. He was forced to step down as acting defense secretary during the confirmation process so the Pentagon can comply with the stipulations of a federal law known as the Vacancies Act.  

The musical chairs is the latest episode in a prolonged period of instability atop the Pentagon. Spencer, a Wall Street financier and former Marine aviator, is the third acting defense secretary to lead the building this year. The department hasn’t answered to a Senate-confirmed leader since Jim Mattis resigned as defense secretary in December over policy disputes with Trump.

In a memo to all military personnel and Defense Department employees on Monday, Spencer explained that he expects to return to lead the Navy as its top civilian official once Esper takes office.

“While my time in this role is anticipated to be brief, I am fully prepared and committed to serve as Acting Secretary of Defense, and I will provide continuity in leadership of the department,” Spencer wrote.

During the confirmation process, Undersecretary of the Navy Thomas Modly is standing in for Spencer to perform the duties of Navy secretary. Ryan McCarthy, performing the duties of Army secretary since Esper was elevated to lead the department, is returning to his role as undersecretary of the Army. McCarthy will then seek Senate confirmation to replace Esper permanently atop the Army.

Senate leaders, who have been clamoring for Trump to install a permanent defense secretary for months, have signaled that they expect a relatively quick confirmation process. Senate Majority Leader Mitch McConnell (R-Ky.) previously said he expected Esper’s confirmation before the August recess.

The effort to fill the Cabinet position faced a setback when former deputy defense secretary Patrick M. Shanahan, who took over for Mattis and received Trump’s backing to become defense secretary, withdrew from consideration over revelations about violent turmoil in his family.

Unlike Shanahan, who spent his career as a Boeing executive, Esper is a longtime denizen of Washington. In addition to his career in uniform, he worked in the Pentagon during the administration of President George W. Bush, advised Republican lawmakers on Capitol Hill, represented the defense industry at a trade association and was a lobbyist for the defense contractor Raytheon.

Esper’s ties to Raytheon, in particular, are likely to be scrutinized during his confirmation hearing.

On Monday, Sen. Elizabeth Warren (D.-Mass.), a presidential candidate, released a letter she sent to Esper that raised concerns about his past ties to the company and stated that he declined in a meeting with Warren last week to recuse from all matters involving Raytheon.

“I am troubled by your unwillingness to fully address your real and perceived conflicts of interest and write to ask that you reconsider your refusal to extend your Raytheon recusal through the duration of your tenure at DoD,” Warren wrote.

Jonathan Hoffman, the chief Pentagon spokesman, said in a statement Monday that defense officials had received Warren’s letter and that Esper spoke with the senator. Esper plans to ensure that any matter involving Raytheon “is screened and referred to another appropriate official,” Hoffman said.