The famed Monroe Doctrine got its beginning as a few paragraphs in the seventh annual message to Congress delivered -- in writing -- by President James Monroe. His Secretary of State, John Quincy Adams, is universally acknowledged as the one who actually formulated the doctrine. In nondiplomatic terms, the doctrine says, "Hands off Latin America, powers of Europe! (Especially you, Spain)."
Of course diplomatic language was used, which JQA, son of a president and soon-to-be president, excelled at. The money quote from President Monroe's annual message to Congress -- we would now call it a State of the Union address, dated December 2, 1823 -- is as follows:
"We owe it, therefore, to candor and to the amicable relations existing between the United States and those powers to declare that we should consider any attempt on their part to extend their system to any portion of this hemisphere as dangerous to our peace and safety. With the existing colonies or dependencies of any European power we have not interfered and shall not interfere.
But with the Governments who have declared their independence and maintained it, and whose independence we have, on great consideration and on just principles, acknowledged, we could not view any interposition for the purpose of oppressing them, or controlling in any other manner their destiny, by any European power in any other light than as the manifestation of an unfriendly disposition toward the United States.
Because the United States had a weak navy at the time, European powers generally ignored the doctrine during the 19th century -- and yet the Spanish and other powers didn't often try to colonize or recolonize Latin America, because it became part of the British Empire's informal sphere. What the U.S. Navy couldn't do (until the time of TR), the Royal Navy did, with the unintended consequence of enforcing the Monroe Doctrine.