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.

Patrick Shanahan, a former Boeing executive who became acting secretary this January, gave up on his nomination after a history of violence between his family members surfaced. Esper, aged 55, is a graduate of West Point and served combat roles in the Gulf War. His career spanned across the government, think tanks and the private sector.