Question

how did U.S. expansion in the late nineteenth century extend and change the principle of manifest destiny

Answer 1

Early American settlers just wanted more land and personal space, so they moved west of the Appalachians. Later, this extended to the belief that it is American destiny to expand to the Atlantic Ocean. 

Answer 2

The manifest destiny idea from the beginning of the 19th century was centered in the conquest of the west. The moral justification was needed for territorial expansion. And as expected, this idea ignored the claims of Native Americans and Hispanics already settled on western lands. Such desire provoked a violent confrontation in the southwestern borders.

By the end of the 19th century, the idea of manifest destiny was related to the American industrialist's need for new markets. Also, the ideology of social Darwinism and religious "crusades" were used to justify the colonization of less developed nations.