Acting U.S. Defense Secretary Mark Esper waits to greet Qatar's Emir Sheikh Tamim bin Hamad Al-Thani at Pentagon in Arlington, Virginia, U.S., July 8, 2019. REUTERS/Yuri Gripas

WASHINGTON (Reuters) - Mark Esper was formally nominated to the position of defense secretary by the White House on Monday, a role he has been carrying out in an acting capacity since last month.

There has been no confirmed defense secretary since Jim Mattis resigned in December, the longest period in Pentagon history. 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.