Why is the Monroe Doctrine significant?
The Monroe Doctrine was a further indication of the United States' intention to remain isolationist and uninvolved in the affairs of Europe. It came about when there was fear that French troops, which had invaded Spain to restore the Spanish king, would attempt to restore the Spanish Empire in the Americas. Previously Russia had attempted to claim lands well below the 51st parallel as part of Alaskan territory. At that point, Secretary of State John Quincy Adams had stated that the America's were closed to further colonization. President Monroe at first considered an alliance with England to oppose any Spanish moves in the Americas but followed Quincy Adams advice and remained neutral. Adams famous quote was:
It would be more candid as well as more dignified to avow our principles explicitly to Russia and France than to come in as a cockboat in the wake of the British man-of-war.
The Monroe Doctrine, announced by President Monrow in a Presidential message to Congress in 1823 allowed the U.S. to avoid committing to an alliance with Great Britain when he knew it was in Britain's interest to avoid restoration of the Spanish Empire.
The doctrine had no weight as international law, and wasn't even called the Monroe Doctrine until 1852. It didn't even bother the European powers enough for any of them to renounce it. It allowed Monroe to flex some American muscle at a time when he knew he could depend on the British navy to keep other European nations out of the Americas.
This is a very good explanation of what the Monroe Doctrine was, but it does not really talk about the significance of the Doctrine moving forward. The Monroe Doctrine (or at least the idea behind it) has had major implications for the Western Hemisphere and for the US's relations with other countries in the area.
For example, the Monroe Doctrine has led to increased American involvement in countries around Latin America and the Caribbean. It has come to mean that the US has the "right" to intervene in the affairs of other countries in the area whenever those affairs might affect the security or interests of the US. You might argue that the Monroe Doctrine played into such things as the US domination of countries like Cuba, Nicaragua, and the Dominican Republic early in the 20th century. You might argue that it plays into things like the Cuban Missile Crisis and the Bay of Pigs invasion.
In other words, the Monroe Doctrine was really saying "this is our sphere of influence and it is our right to try to shape the countries of this area to be more congenial to our interests." This attitude has had major impacts over the last 190 years (especially since the late 1800s).
I agree with post #3, in that the Monroe Doctrine was largely ignored and overlooked in the 1800s. Only after the Spanish-American War and later American imperialism did it become a precedent and justification for further action like the Roosevelt Corollary and Latin American interventions.
There was also some use of it as a justification for regime change in Guatemala, Nicaragua and Chile. More recently, Ronald Reagan cited it directly as justification for invading the island of Grenada in the early 1980s.
I think it serves as a precedent overall, and has been used in ways far removed from what it was first intended to be and do.
Over the years the Monroe Doctrine has become the "hall pass" so to speak allowing the United States to intervene where they feel there is a possible threat to the Democratic government of any country in the Western Hemisphere. Under Theodore Roosevelt the Roosevelt Corollary was added to the Monroe Doctrine stating that any chaos in a small country could lead to intervention from a larger government and that government would be the United States for any smaller country in the Western Hemisphere.
I agree with other editors in that, although it may have initially helped the US to justify is isolationist stance with Europe, the Monro Doctrine then was used to actually give it justification to become involved in greater foreign intervention. It was a Doctrine that helped create the era of the United States considering itself a benevolent superpower helping less developed countries in times of need by foreign intervention.