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.
-President James Monroe‘s 7th Annual Address to Congress 12.2.1823
When the United Colonies, in General Congress Assembled, declared their independence from Great Britain there were three European powers occupying North America: Spain, Russia and England. By the time the 1790′s rolled around, France was reoccupying the Louisiana Territory; a tract of land France had ceded to Spain following the end of the French & Indian War. For her own part, the new United States of America had little means of removing these powerful Europeans from American soil. It had been with the assistance of the French, and a lesser degree the Spanish, that the US had even won her independence. However, the problem of French occupation quickly found a peaceful resolution when Thomas Jefferson authorized the purchase of the Louisiana Territory. Within a decade of that purchase, the United States found herself in a second war with the English; the War of 1812. While this war is still considered by many to be a status quo war, it demonstrated the emergence of American military capabilities.
It was with this that 11 years after the Americans stood toe to toe with the English that President James Monroe promulgated his Monroe Doctrine. This doctrine warned the powers of Europe to never again attempt to colonize the Americas. Yet, our Monroe Doctrine did not take into consideration that in 1823 the United States did not have the military capabilities to enforce this doctrine. Therefore, the Monroe Doctrine relied heavily upon our good relations with the English. American military power was not at the point of enforcing such a doctrine until after the Spanish-American War, which was explicitly fought to enforce the Monroe Doctrine.
The Roosevelt Corollary to the Monroe Doctrine added the next evolution in American military involvement in the world. In addition to preventing European powers from occupying the Americas, the Roosevelt Corollary promised American intervention in Latin Countries unable to pay international debts. It also declared the right of the United States to intervene and stabilize any Latin American Country. This doctrine helped create a partnership between the United States and her Latin American counterparts to the South. It was not, however, the last evolution of the Monroe Doctrine. Rather, the Monroe Doctrine would undergo another change in the late 1940′s.
With World War II officially over, the post-World War world began to take shape. In a matter of years it was apparent that the United States and Soviet Union were settling in for a long, cold War. President Harry S. Truman, hoping to halt the spread of Communism, issued his own corollary to the Monroe Doctrine: The Truman Doctrine. This doctrine stated that the United States would send troops to anywhere in the world in order to prevent the spread of communism. It was under this doctrine that the United States became involved in the Korean War and Vietnam War. As an extension to the Truman Doctrine was the Marshall Plan. The Marshall Plan was the economic side of the Truman Doctrine. The plan called for the United States to economically prop up Western Europe to help confront the Soviet Union.
Finally, the last of the evolutions of the Monroe Doctrine came in the wake of 9/11. The Bush Doctrine called upon the United States to meet the spread Terrorism anywhere in the world it may find safe harbor. This doctrine has resulted in the United States intervening in countries such as Afghanistan and Iraq.
The Monroe Doctrine was devised to simply assert that the United States would not tolerate European intervention in the Americas. Since it was first put forth by James Monroe, the Monroe Doctrine has been transformed to assert American right to intervene in the governments of Latin America, under the threat of Communism, or supporters of Terrorism.