Acting US Defense Secretary Mark Esper waits to greet Qatar's Emir Sheikh Tamim bin Hamad Al-Thani at the Pentagon in Arlington, Virginia, US, July 8, 2019. (Photo: Reuters)

The nomination set off a temporary, but complicated, series of changes for Pentagon leadership. In a statement, the Pentagon said that by law when Esper was nominated on Monday, he stopped serving as acting defense secretary and reverted to his previous job, which was Army secretary. Until Esper is confirmed, Navy Secretary Richard Spencer will serve as acting defense secretary.

Esper took over as acting defense secretary in June when Patrick Shanahan abruptly quit after having served as acting secretary since Mattis departed. Thus Spencer, filling in for Esper as of Monday, is the third acting defense secretary this year.