The Monroe Doctrine is a key part of U.S. foreign policy. President James Monroe issued the policy in 1823. It stated that North and South America were no longer open to colonization. It also declared that the United States would not allow European countries to interfere with independent governments in the Americas.
From what I know, the Monroe Doctrine was made to prevent further British colonization of territories in the Western Hemisphere. Through the process of elimination, it's D. Terribly sorry if I'm wrong.
The Sparta, Athens, and other Greek communities collected natural resources to build walls to protect them from external attacks. However, they eventually succumbed to attacks via the ocean route.
