Who is John D Long from The Rise of Theodore Roosevelt and what is their importance?

As assistant secretary of the navy, Roosevelt serves under John D. Long, a career politician in his later years who works as little as possible. He easily gives control over to Roosevelt so he may go on summer vacation. Like President McKinley, Long desires to keep peace at a high cost, a philosophy that Roosevelt hates. However, after the sinking of the Maine, Long supports Roosevelt's suggestion to go to war.