Department of Defense military land use of the desert southwest includes a wide spectrum of military weapons testing, force-on-force training, and various types of flight training. The desert southwest provides a critical asset for the U.S. military— open space. Installations in the desert southwest tend to be much larger than installations in other regions of the nation, with several exceeding 400,000 ha. This open-space asset has allowed the military to historically establish large training areas and ranges on installations and to define expansive air maneuver regions above these ranges and above the vast public lands of other agencies. It also offers critical training and testing areas that are analogs to similar worldwide environments where the military operates. Training and testing activities are conducted in the three-dimensional land and air space that replicates the modern battle space. Land and air space use is highly variable among installations depending on mission requirements. Natural resource management challenges include the large spatial extent of lands and air space under Department of Defense management, highly variable military land-use requirements, significant endangered species regulatory and conservation requirements, encroachment and Base Realignment and Closure requirements, competition for water resources, and climate change. Department of Defense natural resource managers attempt to meet these challenges through interagency cooperative agreements, integrated natural resource management plans, and Department of Defense sustainable range programs.