Simplify the following text for a Grade 6 native English-speaking person.
Original: The Marine Corps remained as a separate service under the Department of the Navy , and the Coast Guard was under Department of Transportation , ready to be shifted to the Navy Department during time of declared war ( as it was in both world wars ) .
Simplified: ( The Department of the Navy includes the Navy and the Marine Corps . Also , during time of war the United States Coast Guard is under the authority of the Department of Defense . )