The

a0 Doctrine stated that European nations should not interfere with or acquire more territory in the Western Hemisphere.

It's the Monroe Doctrine.

Explanation:

In a speech before Congress, president James Monroe said that the United States would treat as a hostile act any interference of European powers in the affairs of American nations, and that the US would not get involved in the affairs of Europe. The US saw in the possibility of European meddling a security threat. This is the Monroe Doctrine. It was formulated in 1823, many Latin American republics had won independence in recent years, and there were fears that either Spain or France could try to regain control or build influence on the continent.