Answer:
The Manifest Destiny was a doctrine that believed that the US expansion towards the west was inevitable, and that it was to spread American beliefs, values, and culture to the west (or to present-day California, essentially). It took on a moral principle to justify what would be commonly known as an invasion, to bring the whole country into the beliefs of Christianity. The Manifest Destiny continued on until around the American Civil War, where it was dropped for the more pressing issue of the freedom of slavery as well as the reunification of the United States on the northern front, and the ability to continue to keep their slaves and state's rights on the southern.
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