This is a great question. While United States Americans are taught about 'manifest destiny' in school (a very whitewashed, biased version), and it was a concept that greatly shaped the United States - both ideologically and by international borders - it is truthfully only the continuation of a European tradition - the same tradition that inspired the very founding of the first European colonies in North America and their associated conflicts, such as the Anglo-Powhatan Wars and Pontiac's War, and continues on in insidious ways today, especially apparent now in the Levant, Afghanistan, and even the South China Sea. In schools in the United States, we are taught that 'manifest destiny' was the moniker of a long-ago zeitgeist, something that doesn't really affect us anymore, but that is very far from the truth. It is only a poetic pseudonym for the spirit of a resurgent United States American imperialism. The ideology as it was when it was defined was wholly, unapologetically racist and fostered by racist white United States American columnists and pedagogues. The racism and the peddling of imperialism practised these days by the commentariat is only less outright, relying on dog-whistles and suggestion. It was a period of great, bombastic violence, such as the Trail of Tears and the war against Mexico, that cost many non-white communities their lives, land, and cultures. Now United States imperialism is more strategic or much farther from the continent, and thus much easier to ignore. But, as I'll repeat one last time, it has never left the U.S.. While it may have been imported from Europe alongside a culture of war and conquest, what would eventually be known as 'manifest destiny' became a monster in its own right, and would most notably be exported back into Europe and interpreted as Lebensraum by the German NSDAP. It looks like I've run out of space to write but if you have any questions about what I wrote, feel free to ask and I will be happy to answer!

As an American I hope that I am qualified to answer your question. "Manifest Destiny" in essence was the desire to spread the "American Civilization" from the Atlantic all the way to the Pacific. After the American Revolution, many of the colonies were still regarding themselves as separate entities and manifest destiny meant the unity of those colonies as well as the creation of a unique culture separate from European origins. Americans were just gaining a sense of independence and (unfortunately) a sense of superiority over other cultures, cultures that included Native Americans, Mexicans and others. The early Americans considered themselves "God's people" much in the way that Hebrews (Jewish peoples) believe that they are God's people and Israel is bequeathed to them by god, so did Americans believe that the current US territory was intended for them by God. They also believed that the American civilization was unique and intended for the good of all man kind. You can see how the beliefs above still resonate with a large portion of the population of the US. Many people still think of the US as "God's Gift to Man," the result is an arrogant sense of superiority as well as innate correctness. I think we Americans would do better to question ourselves and our actions but unfortunately I think many Americans would find that painful.

Part 4 and last. So, into these conditions, you have the young United States, eager to become an important power. To the west there is all this land. Yes, there are claims on it from the Spanish, British, and French... and there are people living on it already, but their claims weren't seen as legitimate. The country had to decide what its borders should be, and how much of its resources to devote to buying, negotiating treaties, or capturing the land to make it possible. Manifest Destiny was the idea that the young nation had the power--financial and military--to acquire all the land to the West Coast, which among other things would be a good militarily defensible boundary... phrased in terms that suggested we were (somehow) entitled to this land by some mysterious natural right.

Part 1 of several... Where do any national cultural beliefs come from and how do they persist? I won't try to answer your question directly but I'll mention some things that _may_ help. 1) Nobody talks about "manifest destiny" except as a piece of history. It's a phrase we learn from history books. On the other hand, although people don't normally use the phrase, "American exceptionalism" still exists. This is the idea that the United States is somehow different from other countries, and that we are specially favored, specially lucky, and entitled to a special place in the world. Some of this is just ordinary national arrogance. Some of it does have something to do with our national history: the unusual situation of building a brand-new nation. 2) I'm going to mention two things my mom said. I don't agree with either. She died in the 1970s and she might not agree with them today, either. But they show the kind of ideas people had, as recently as fifty years ago. The first is intentionally ironic: "The reason Europeans look down on us is because we bought our land, instead of stealing it." What she meant was that for centuries European nations had been fighting wars among each other and taking land away from each other, while the United States bought nearly 1/3 of its land in purchases--the big ones being the Lousiana Purchase, from France, and Alaska, from Russia. Of course you might ask if France really owned that land and really had the right to sell it to us. The "Indians"--the Pawnees, Arapaho and Sioux nations would say no! But it is nevertheless a U.S. cultural idea that we obtained our land legally.

There is no way to answer such a complex question with a few words. Manifest Destiny today is probably only interesting to a few historians. This article on Wikipedia seems to be a good examination of the topic: https://en.m.wikipedia.org/wiki/Manifest_destiny I didn't read the entire passage so you'll have to draw your own conclusions. Good luck. You have a big job ahead of you.