What did the Monroe Doctrine state?

The Monroe Doctrine declared that any intervention by external powers in the affairs of the Americas would be viewed as a potentially hostile act against the United States. This principle was rooted in a desire to protect the newly independent nations of Latin America from European colonial domination and influence. It asserted that the Western Hemisphere was no longer open to colonization and that any attempt by European nations to interfere in the political matters of the Americas would be met with resistance from the United States. This doctrine established a foundational component of U.S. foreign policy for many years, emphasizing the importance of American sovereignty in the region and signaling a shift towards a more assertive role in Western Hemisphere affairs.

The other options do not accurately reflect the content of the Monroe Doctrine. The assertion of neutrality in European conflicts is more aligned with earlier American foreign policy but is not a primary focus of the Monroe Doctrine. The claim about assisting European countries in colonization directly contradicts the doctrine's intent to oppose European colonial ambitions. Lastly, the idea that new states could not join the Union is unrelated and does not pertain to the principles established by the Monroe Doctrine.