The Marines have been aiding the State Department in one way or another since the 18th century, likely in large part because Marines were highly trained warriors who served aboard Navy ships -- and in the 18th and 19th century, how did one get to a diplomatic post from the United States in the era before air travel? Via naval assets. Armed naval assets that could provide protection during the journey and once they arrived.

Leo J. Daugherty III wrote a comprehensive book on the history of the partnership: "The Marine Corps and the State Department: Enduring Partners in United States Foreign Policy, 1798-2007".