Complex sentence: Japan had already been a concern of the U.S. Navy--strategist Admiral Alfred Thayer Mahan had warned then-Assistant Secretary of the Navy Theodore Roosevelt in 1897 of much more likelihood of conflict in the Pacific than in the Atlantic.
Split into simpler sentences: Japan had already been a concern of the U.S. Navy. Strategist and Admiral Alfred Thayer Mahan had warned then-Assistant Secretary of the Navy Theodore Roosevelt in 1897 of a much greater likelihood of conflict in the Pacific than in the Atlantic.