The Monroe Doctrine is all about power. Not like Incredible Hulk power, or Louis XIV power, but a less in-your-face type.
President Monroe challenged centuries of colonization by European nations, whose only opponents had been each other. However, he doesn't just say "Down with Europe, there's a new sheriff in town." Monroe recognizes the power that Europe has in the Americas, promises not to interfere, while also clearly drawing a line in the sand.
He doesn't totally upset the balance of power, but does declare the U.S. to be a new contender in the sort of real-life Game of Thrones (without all the dragons and beheadings…as far as we know).
By being the first to successfully fight for independence, the United States was able to take a power position in the Americas, having a head start in the whole "building a nation" thing. That's why Monroe was able to issue the Monroe Doctrine, and why later presidents could wield its power somewhat effectively.
Monroe's repeated assurances to not interfere with European colonies shows the reality of America's weakness at the time. He knew there was only so far he could go.
The Monroe Doctrine is like the poster child for American Exceptionalism. The idea that the U.S. has a unique ideology and a special mission to spread that ideology in the world found a real foothold in this speech.
The president directly talks about the American system vs. the European one, but there is also an undercurrent of moral superiority throughout the text. The implication is that America is a better guardian of Latin America than Europe was; also, any attempt to re-take Latin America would obviously only happen as part of a threat to re-take the U.S. as well. It's all about the United States, even when it's not.
The Monroe Doctrine also paved the way for Manifest Destiny, an even better poster child. Not that it's a competition—everyone gets to pose for that American Exceptionalism poster.
The fact that Monroe thought this part of his speech was a good idea shows just how highly Americans thought of themselves. He's potentially taking on powerful European countries despite having almost no military power; that's the sign of someone extremely confident with their image.
Although the Monroe Doctrine isn't super obvious about spreading the American system far and wide, it opens that door, which leads to the future addition of the rest of the United States. And Puerto Rico…and Guam…and American Samoa.
From the 16th century onward, building empires is a part of everyday life for a number of European countries.
With the Monroe Doctrine, the U.S. deflected that imperialism away from the Americas. It also, without explicitly saying so, established the United States' own form of imperialism: expansion into the west, with an occasional takeover of a country here and there (hola, Puerto Rico).
Monroe's words include a combination of fear and determination, all in regards to the fate of territory in the Americas. To colonize or not to colonize? That apparently is the question.
When Monroe gave the speech containing the Monroe Doctrine, he really was just trying to avoid more European power in the Americas. That being said, later leaders used that protectiveness as a reason for their own special brand of imperialism.
Since all the rumors of Europe wanting more American colonies were pretty much wrong, the Monroe Doctrine didn't really stop anything from happening until later in the century. So really, it had a kind of delayed effect with regard to imperialism.
The strength and purpose of the Monroe Doctrine relies on there being a difference between the U.S. (with Latin America) and Europe. The idea of America as the pure republican improvement to the corruption of absolutist Europe goes back to the pre-Revolution days, and persists throughout the 19th century.
It's New World versus Old World, republic versus monarchy, former colonies versus colonizers, Gryffindor versus Slytherin.
Monroe keeps his language very diplomatic and respects Europe's current territory, but he also tries to strengthen the natural border that is the Atlantic Ocean. In the immortal words of the Backstreet Boys, "'cause we / are two worlds apart / can't reach to your heart."
Monroe had to play up the division across the Atlantic Ocean to justify the fact that he didn't want any more European colonies, without being too warlike about it. It's like he's trying to convince everyone that they've just grown too far apart, and things will never be like they once were, so everyone has to move on with their lives.
The Americas were, in fact, very different from Europe in a whole bunch of ways, because of their history and location. As the rest of the century proves, the contrasts between the Americas and Europe did make any further colonization a pretty bad idea.
Even in the 1820s, politics were pretty global. Relationships between nations and their rulers started wars, dictated trade agreements, and affected territory disputes. Colonialism had spread the web of European politics in particular across the world, and the more powerful countries were always watching out for the balance of power.
Yeah. Politics has always been messy.
The Monroe Doctrine was a potential thorn in the side of the big European political players. Monroe uses political knowledge and some very diplomatic language to be strong in his statements without ignoring the political reality. He knew to be clear that defending the new republics did not mean messing with existing colonies, or meddling with power struggles on the European continent.
Monroe had to keep up political appearances, because really he couldn't physically do anything if Europe wanted to invade Latin America in 1823.
The Monroe Doctrine might be seen as an addition to the Constitution or the Declaration of Independence, by adding a protective bubble as America continued to expand. The U.S. had already established how awesome its form of government was, now it was time to make sure no one threatened it.