Final answer:
Each military department in the United States is led by its own civilian secretary, upholding the constitutional principle of civilian control over the military.
Step-by-step explanation:
Each military department—army, navy, and air force—is led by its own civilian secretary. This reflects the United States Constitution's principle that the military is under civilian control. The highest-ranking military officers, such as the Joint Chiefs of Staff, must report to these civilian leaders and ultimately to the President, who is the Commander in Chief. The civilian secretaries are part of the President's Cabinet, and they play a crucial role in overseeing the various branches of the military and advising the President on military affairs.