The doctrine that provided a rationale for the westward expansion of the united states in the 19th century

Manifest destiny
Was a widely held belief in the United States that its settlers were destined to expand across North America. Championed by president Polk, the USA based the manifest destiny on three themes, such as special virtues,and the essential duty of the American people to redeem the west given by God.

The correct answer is Manifest Destiny.

Manifest Destiny is the belief that it is the god given right for America to expand their power from "sea to shining sea." The sea to shining sea refers to the land between the Atlantic and Pacific Ocean.

This term coined by John O' Sullivan helps to explain why America bought land from other countries (like France and Spain) and fought wars against other countries (like Mexico) during the 19th century. Overall, Manifest Destiny helped the US gain numerous states like Texas, Florida, Arizona, California, Colorado, Utah, New Mexico, etc.