The Monroe Doctrine was a very important document. President Monroe issued it in 1823. It told Europe they could have no new colonies in the Americas. Many countries in Central and South America got their independence in the early 1800s. The Europeans were considering trying to invade these countries to take away their independence. To prevent this, we issued the Monroe Doctrine. However, since we weren’t strong enough to enforce the Monroe Doctrine, we depended on the British to do this. The British were more than willing to do this because they had few possessions in the Americas, unlike their rivals. Thus, helping us was a way for Britain to weaken its enemies. It also established our presence throughout the Americas.

In the early 1900s, President Theodore Roosevelt issued the Roosevelt Corollary to the Monroe Doctrine. This told Europe that if they had issues with Latin American countries or countries in the Caribbean, we would handle them. We reinforced the idea that Europe would not intervene in affairs of countries in the Americas. An example of the use of this corollary was in 1905. We intervened in the Dominican Republic when they fell behind paying their debts to Europe.

Both the Monroe Doctrine and the Roosevelt Corollary to it reinforced the idea that Europe would not interfere with the affairs of countries in the Americas.

In his December 2, 1823 address before Congress, President James Monroe emphasized that the United States would seek to negotiate with other powers, especially with Britain, Spain and Russia -- the latter's claims over significant portions of the western coast along the Pacific were predicated upon its attempts to colonize those territories -- the proper adjudication of territorial disputes, but that the continental United States, as it then-existed, was not subject to such negotiations, and that European colonization of the Americas was no longer open to discussion: 

"The Government of the United States has been desirous by this friendly proceeding of manifesting the great value which they have invariably attached to the friendship of the Emperor and their solicitude to cultivate the best understanding with his Government. In the discussions to which this interest has given rise and in the arrangements by which they may terminate the occasion has been judged proper for asserting, as a principle in which the rights and interests of the United States are involved, that the American continents, by the free and independent condition which they have assumed and maintain, are henceforth not to be considered as subjects for future colonization by any European powers."

What subsequently became known as "the Monroe Doctrine" has since that time been interpreted to mean the United States viewed the Americas as within its sphere of interests and that foreign -- read: European and Asian -- intervention in the Americas would contravene those U.S. interests.  The Monroe Doctrine has, consequently, been used to define U.S. national interests and has served as a warning to other powers, mainly, to the Soviet Union during the Cold War and, potentially, to China in the not-too-distant future, that any military and political activities in the Americas should remain limited in scope, and that no actions taken in the Americas should constitute a threat to U.S. interests.  

There are two main significances of the Monroe Doctrine.

The first significance was felt immediately.  This was the improvement of relations between the United States and Great Britain.  The two countries had had a very tense relationship after the Revolutionary War.  This tension included the War of 1812.  However, the US and Britain cooperated on the formulation of the Monroe Doctrine, thus improving their relations.  This was important for the United States as it was a relatively weak country and Britain was a superpower.

The second significance did not come until much later.  As the US became more powerful, the Monroe Doctrine established its “right” to exercise its influence over all of the Americas.  This allowed it to do such things as threatening to invade Mexico to drive out the government of Emperor Maximilian.  It allowed the US in the early 1900s to have a basis for occupying countries like Haiti and the Dominican Republic.  In short, it gave the US a foundation that allowed it to exert a great deal of control over many of the countries of Latin America.