When the Navy Department was founded in 1798 its primary mission was to protect American merchant ships. A century and a half later, the National Security Act of 1947 was the authority for the roles of the U.S. Navy.
Landersman, S. D.
"Naval Protection of Shipping: A Lost Art?,"
Naval War College Review: Vol. 39
, Article 4.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol39/iss2/4