Final answer:
The Monroe Doctrine was initially a strategy to assert that Europe should not colonize or interfere in the Western Hemisphere, while the Roosevelt Corollary was an expansion that allowed the U.S. to intervene in Latin American affairs to maintain order and stability.
Step-by-step explanation:
The Monroe Doctrine initially described a strategy designed to promote the independence of countries in the Western Hemisphere and discourage European interference. Established in 1823 during President Monroe's term and with significant input from Secretary of State John Quincy Adams, its main message was that any efforts by European nations to colonize land or interfere with states in North or South America would be viewed as acts of aggression, requiring U.S. intervention. The doctrine asserted that the Western Hemisphere was not to be further colonized by European countries, and that the political system of the Americas was distinct from Europe.
The Roosevelt Corollary, introduced by President Theodore Roosevelt in 1904, built upon the Monroe Doctrine. It asserted the United States' right to interfere in the economic affairs of neighboring countries in the event of chronic wrongdoing, effectively positioning the U.S. as an international police power. This corollary justified American intervention in Latin America to stabilize economic affairs and was seen by many as an extension of an American imperialistic agenda.
While the Monroe Doctrine established a protective stance toward Latin American countries against European colonization, the Roosevelt Corollary shifted that stance to one of more active intervention, allowing the United States to assert dominance in the Western Hemisphere, often under the guise of maintaining stability or providing assistance.