The Monroe Doctrine and the Roosevelt Corollary
The Monroe Doctrine, put forth in 1823 by President James Monroe, called for an end to European intervention in the American continents (both north and south). This applied only to independent governments in the Americas however, not to areas that were colonies at that time.
In what came to be known as the Roosevelt Corollary to the Monroe Doctrine, Roosevelt asserted that European nations should not intervene in countries to the south of the US, however under certain conditions, United States intervention might be justified.
“There are certain essential points which must never be forgotten as regards the Monroe Doctrine. In the first place we must as a nation make it evident that we do not intend to treat it in any shape or way as an excuse for aggrandizement on our part at the expense of the republics to the south. We must recognize the fact that in some South American countries there has been much suspicion lest we should interpret the Monroe Doctrine as in some way inimical to their interests, and we must try to convince all the other nations of this continent once and for all that no just and orderly government has anything to fear from us. There are certain republics to the south of us which have already reached such a point of stability, order, and prosperity that they themselves, though as yet hardly consciously, are among the guarantors of this doctrine. These republics we now meet not only on a basis of entire equality, but in a spirit of frank and respectful friendship, which we hope is mutual. If all of the republics to the south of us will only grow as those to which I allude have already grown, all need for us to be especial champion of the doctrine will disappear, for no stable and growing American republic wishes to see some great non-American military power acquire territory in its neigborhood. All that this country desires is that the other republics on this continent shall be happy and prosperous; and they cannot be happy and prosperous unless they maintain order within their boundaries and behave with a just regard for their obligations toward outsiders. (Fifth Annual Message, Washington, December 5, 1905.) Mem. Ed. XVII, 352-353; Nat. Ed. XV, 301-302.