Final answer:
The Coast Guard is the military service not controlled by the Department of Defense in peacetime, functioning instead under the Department of Homeland Security.
Step-by-step explanation:
Which Military Service Is Not Controlled by the Department of Defense in Peacetime?
In peacetime, the military service that is not controlled by the Department of Defense is the Coast Guard. The Coast Guard is part of the Department of Homeland Security in peacetime, although it can be transferred to the Department of the Navy during times of war or by direction of the President of the United States. This unique arrangement ensures the Coast Guard's ability to perform both military and law enforcement duties that contribute to national security and public safety.
Article 2 of the Constitution establishes that the President is Commander in Chief of the armed forces, ensuring civilian control over the military, including the Coast Guard. During peacetime, the Coast Guard conducts a variety of missions, including search and rescue, maritime law enforcement, and maintaining navigational aids.