What did Alfred Mahan believe was essential for America to have to protect overseas trade?
Alfred Mahan was a naval officer who studied large empires and found that empires such as Great Britain would not have gained there strength without the navy. He published his findings in the book, The Influence of Sea Power Upon History. He thought navy wasn't just important for trade but also for the U.S. as a world power and an empire.
It should also be noted that Theodore Roosevelt took this to heart and expanded the Navy while he was president.