The Monroe Doctrine is an American Government policy statement of nonintervention that has had wide-ranging if sporadic enforcement and uses since it was introduced.

Named after President James Monroe, who announced it in a speech on December 2, 1823, the Doctrine warned European countries against colonizing or interfering with the governments of countries in North or South America, promising American intervention as a response.

"With the movements in this hemisphere we are of necessity more immediately connected, and by causes which must be obvious to all enlightened and impartial observers. The political system of the allied powers is essentially different in this respect from that of America. This difference proceeds from that which exists in their respective Governments; and to the defense of our own, which has been achieved by the loss of so much blood and treasure, and matured by the wisdom of their most enlightened citizens, and under which we have enjoyed unexampled felicity, this whole nation is devoted. 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."

The Doctrine also contained a promise that the U.S. would not interfere with existing European countries or colonies:

"In the wars of the European powers in matters relating to themselves we have never taken any part, nor does it comport with our policy to do so."

In effect, Monroe was drawing an invisible line of demarcation between the New World and the Old World, in terms of spheres of influence. At the time, not many European colonies were left in the New World. Thanks to the efforts of Simon Bolivar and others, nearly all Latin American colonies of Portugal and Spain had declared themselves independent from their colonial "parents." Two exceptions very close to U.S. borders were Cuba and Puerto Rico.

After the end of the Napoleonic Wars, Austria, Prussia, and Russia formed the Holy Alliance, dedicated to promote monarchic governments. These efforts included plans for helping Spain re-establish its sovereignty over some of the very Latin American countries that had just won their independence.

Secretary of State John Quincy Adams played a large part in what came to be known as the Monroe Doctrine. He signaled concern of the possibility of renewed European influence over Caribbean and South American territory to American ministers in Great Britain, Russia, and Spain. British Foreign Minister George Canning was very receptive to some sort of declaration of authority between his country and the U.S. Britain was enjoying a profitable trade agreement with the United States at the time, and neither country wanted to see other European countries regaining footholds in the New World.

The result was Monroe's pronouncement, which was largely disregarded by other European countries but was enforced by the British Navy. (Although the Monroe Doctrine stipulated that the U.S. would respond to any European invasions in the New World, the American Navy at the time wasn't a large one.)

The Monroe Doctrine also included a warning to Russia, which had been sending ships to Alaska in search of territory for colonization. (The U.S. eventually bought the Alaska Territory from Russia in 1867.)

As the American military grew in size and stature, the U.S. had occasion to cite the Monroe Doctrine in conflicts between nations. One of the first such instances was a warning to Great Britain, to stay away from Hawaii. President John Tyler began the process of annexing the islands in 1842. The U.S. also found use for the Monroe Doctrine when French forces conquered Mexico in 1862. This was the first widespread use of the term. American forces were unable to intervene in Mexico initially because of the Civil War and entered the area belatedly a few years later. Mexican forces threw off the rule of Emperor Maximilian themselves, on Cinco de Mayo.

The Monroe Doctrine has evolved through the years. At the time of its introduction, it was viewed as a statement of American prominence. After the American victory in the Spanish-American War and a severe crisis in Venezuela threatened a wider war, President Theodore Roosevelt added the Roosevelt Corollary to the Monroe Doctrine. This Corollary, announced in 1904, authorized American forces to intervene in Latin America if they found “flagrant and chronic wrongdoing by a Latin American Nation.” The U.S. Government cited the Roosevelt Corollary as a means for intervening militarily in disputes in Haiti and the Dominican Republic.

Since then, it has been invoked to justify interventions in Latin America that were not always welcome. One of the last revisions of the Doctrine was by Cordell Hull, who was Secretary of State for a time under President Franklin D. Roosevelt. In 1933, Hull signed a protocol asserting that the U.S. would follow a policy of nonintervention.

Conversely, President John F. Kennedy cited the Monroe Doctrine in the tense negotiations surrounding the Cuban Missile Crisis.