This refers to the fact that Monroe was the last US President to have served in any significant capacity in the American War of Independence.  The first five Presidents all played significant roles in the Revolution. George Washington, John Adams, Thomas Jefferson, James Madison, and James Monroe are known as the cocked hats.  Of the five, three (Adams, Jefferson, and Madison)  played a significant role in defining the philosophical underpinnings of this country, and two (Washington and Monroe) served in the new nations armed forces.

Five stars for truism.