Complex sentence: ONI grew in prominence under President Theodore Roosevelt, a former Assistant Secretary of the Navy and naval enthusiast, whose expansionist foreign policy--and the central role the U.S. Navy played therein--made ONI's intelligence apparatus more crucial.
Split into simpler sentences: ONI grew in prominence under President Theodore Roosevelt, a former Assistant Secretary of the Navy and naval enthusiast. His expansionist foreign policy--and the central role the U.S. Navy played therein--made maritime intelligence more crucial.