The primary factor in the spread of U.S. westward expansion was the ideology of Manifest Destiny, which held that Americans were destined to spread across the continent. This belief encouraged settlers to move westward, resulting in the acquisition of vast territories through treaties, purchases, and military conquest. The discovery of gold in California and other resources also spurred migration, leading to the establishment of new states and a diverse population. This expansion had significant cultural, economic, and political implications, often disregarding the rights of Native American tribes and leading to their displacement and disruption of their societies.