The United States Army Corps of Engineers (USACE) is an engineer formation of the United States Army that has three primary mission areas: engineer regiment, military construction, and civil works. The day-to-day activities of the three mission areas are administered by a lieutenant general known as the commanding general/chief of engineers. The chief of engineers commands the engineer regiment, composed of combat engineer army units, and answers directly to the chief of staff of the army. Combat engineers come from throughout the service and can be active duty, national guard, or army reserve. Combat engineers' duties are to construct fighting positions, fixed/floating bridges, and obstacles and defensive positions, place and detonate explosives (sappers), conduct operations that include route clearance of obstacles and rivers, prepare and install firing systems for demolition and explosives, and detect mines. For the military construction mission the commanding general is directed and supervised by the assistant secretary of the army for installations, environment, and energy, whom the President appoints and the Senate confirms. Military construction relates to construction on military bases and worldwide installations.