To begin with, The Monroe Doctrine was a policy that dealt with the idea of opposing the European colonialism of Americas.
James Monroe formulated the basic principles of the doctrine during his annual message to the Congress. They were as follows: the United States of America would not take part in wars that happened on the European territory; the USA refused to interfere with any colonies that existed in the Western Hemisphere at that time as well as the closing of any future colonization of the Western Hemisphere. What is important to highlight is that the Monroe Doctrine became a turning point in the foreign policy as well as the course which the United States of America took in terms of their international relationships with other countries.
It s clear that the document was developed and broadened after 1870. What should also be mentioned regarding the issue in question is that the Monroe Doctrine became a very influential tool later on due to the fact that the United States of America managed to become a country with world power.
It should also be mentioned that the Latin American reaction to the Monroe Doctrine was rather favorable. However, there were some people who questioned the intent of the USA in regards to the doctrine. As a result, a lot of people highlighted the importance of being alert for the nations of Latin America.
Taking everything into account, the main reason why the Monroe Doctrine was such a significant document in the history of the United States of America is that it demonstrated the desire of the country to resist the power of European colonialism.