Final answer:
The Roosevelt Corollary adjusted the Monroe Doctrine, allowing the U.S. to act as an international police power in the Western Hemisphere to prevent European intervention.
Step-by-step explanation:
The long-standing American policy that Roosevelt adjusted with the Roosevelt Corollary was the Monroe Doctrine. President Theodore Roosevelt extended the Monroe Doctrine with his corollary in 1904, which declared that the United States could intervene in the affairs of independent nations throughout the Western Hemisphere as an international police power, particularly when those nations faced financial instability or political turmoil. This adjustment was made to prevent European nations from intervening in Latin American countries, which would be a violation of the Monroe Doctrine's original intent, and to assert the United States' dominance over the Western Hemisphere.
The Roosevelt Corollary became a tool for the United States to establish protectorates and manage foreign territories' affairs, exemplified through actions in Cuba, Panama, and the Dominican Republic.