President Trump officially nominated Mark Esper as defense secretary on Monday. The position has been in limbo since Gen. Jim Mattis resigned last December, marking an unprecedented period of instability at the Pentagon. Esper has held the position in an acting capacity since June, after he took over from the previous acting secretary, Patrick Shanahan. Shanahan withdrew his nomination after reports emerged of domestic violence within his family. Esper, a Gulf War veteran and former lobbyist for Raytheon, a major defense contractor, is entering the administration amid rising tensions between the United States and Iran. Esper’s Senate confirmation hearing is reportedly scheduled for Tuesday, and he could be confirmed as early as Thursday.