The difference between the Monroe Doctrine and the Roosevelt Corollary lies in the latter's extension of the former to suggest a more proactive, interventionist policy on the part of the United States towards the nations of Latin America.

The Monroe Doctrine was President James Monroe's warning to the nations of Europe against expanding their imperialist practices across the Atlantic. On December 2, 1823, Monroe, in a speech before Congress, addressed the still-new United States of America's continued weariness of European colonial intrusions into the Western Hemisphere. Within the context of U.S. negotiations with Russia and Great Britain regarding the latter two countries' histories of territorial expansionism in North America, and expressing the desire on the part of the United States to establish and maintain cordial relations with both empires, President Monroe nevertheless issued the following admonition against European colonialism in the Western Hemisphere:

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.

The Monroe Doctrine served as the main U.S. policy on European relations in the Americas well into the 20th Century -- its greatest test, perhaps, involving the Soviet Union's attempts at cultivating relationships with certain Latin American countries during peak periods of the Cold War, much to the consternation of the United States. If the Monroe Doctrine was essentially defensive in nature, acting as a deterrent against outside meddling in the United States' sphere of influence, future U.S. President Theodore Roosevelt would fundamentally alter that understanding and inject a more interventionist angle into American foreign policy with respect to Latin America. Known as "the Roosevelt Corollary," this extension of the Monroe Doctrine imposed a more aggressive approach to the Americas into U.S. foreign policy. In his annual message to Congress, on December 6, 1904, President Roosevelt warned, not so much Europe, but the disparate nations of Latin America themselves against developments deemed antithetical to U.S. interests:

If a nation shows that it knows how to act with reasonable efficiency and decency in social and political matters, if it keeps order and pays its obligations, it need fear no interference from the United States. Chronic wrongdoing, or an impotence which results in a general loosening of the ties of civilized society, may in America, as elsewhere, ultimately require intervention by some civilized nation, and in the Western Hemisphere the adherence of the United States to the Monroe Doctrine may force the United States, however reluctantly, in flagrant cases of such wrongdoing or impotence, to the exercise of an international police power.

The Roosevelt Corollary, then, was tantamount to the establishment of a United States' "right" to intervene in the internal affairs of the independent, sovereign nations of Latin America whenever it deemed fit. The suggestion that the United States, or any other great power, would declare itself entitled to intervene in other nations to address political instabilities or to prevent the ascension of governing regimes that were hostile to that power would prove interestingly prescient. To conclude, the fundamental distinction between the Monroe Doctrine and the Roosevelt Corollary was the latter's declaratory expansion of the former into a more interventionist doctrine.