Simplify the following text for a Grade 16 native English-speaking person.
Original: The United States Department of War , also called the War Office , was the department of the United States government 's executive branch responsible for the operation and maintenance of land ( and later air ) forces from 1789 until September 18 , 1947 , when it became part of the National Military Establishment , renamed on August 10 , 1949 the Department of Defense .
Simplified: The War Department existed from 1789 until September 18 , 1947 , when it split into Department of the Army and Department of the Air Force and joined the Department of the Navy as part of the new joint National Military Establishment ( NME ) , renamed the United States Department of Defense in 1949 .