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A significant theme of the Monroe Doctrine was that...
B. European powers should refrain from further colonization in the Americas.
The Monroe Doctrine was perhaps the most notable achievement of James Monroe's presidency. In it, Monroe declared the Americas off limits from European settlers which included both North and South America. This was to establish the land, and the countries in them, as independent of European rule and was a crucial step towards establishing the U.S. as a power.