Manifest destiny was a widely held cultural belief in the 19th-century United States that American settlers were destined to expand across North America. Historians have emphasized that "manifest destiny" was a contested concept Democrats endorsed the idea but many prominent Americans (such as Abraham Lincoln, Ulysses S. Grant, and most Whigs) rejected it. Historian Daniel Walker Howe writes, "American imperialism did not represent an American consensus; it provoked bitter dissent within the national polity … Whigs saw America's moral mission as one of democratic example rather than one of conquest."
The philosophy drove 19th-century U.S. territorial expansion and was used to justify the forced removal of Native Americans and other groups from their homes. The rapid expansion of the United States intensified the issue of slavery as new states were added to the Union, leading to the outbreak of the Civil War.
In the sentence, "to harbor" mean to provide protection to somebody. The actual meaning of harbor refers to a place where ships and boats can be docked in the water. There is no water body or ocean present around Afghanistan so the actual meaning of harbor does not apply. So that's why the harbor word is used for giving protection.
