The romanticized idea of westward expansion and manifest destiny in the minds of colonial settlers led to massive policy changes that greatly disadvantaged Indigenous people as well as enslaved people. This westward push was fueled by media and cultural understandings that the parts of the American continent that hadn’t become a part of the United States were empty, desolate wastelands devoid of people and begging to be ‘civilized’. This notion, combined with the large swaths of land acquired by the United States in the Louisiana Purchase in 1803 and the Mexican-American War ending in 1848, led to policies that pushed Native people out of their land and expanded settler colonialism and slavery further West.
One key example of this might be the fall of Comancheria, which seems to have been destined to care for much of the lands in the Southern Plains in what is now Texas and a few other states in the Southwest. The Comanche people had control over the economy and territory of much of this area and were successful in controlling the flow of goods between the United States and Mexico. However, this control came to an end with the Mexican-American War and many subsequent treaties and policies that pushed Comanche communities into reservations in Oklahoma and stripped them of their power while making room for white settlers and plantation owners to move in and expand the institution of slavery. This history makes it clear that white settlers were not manifesting some great pre-determined, earned destiny but rather seizing land and pushing people into the margins of history for the sake of profit.
The same can be said of California both before and after the Gold Rush. Initially, settlers had already begun to flood into the state via overland trails, leading to ‘military governors’ entering the state and managing ‘Indian affairs’ on behalf of the federal government. Native people were restricted and exploited, with their labor being used by ranchers and their movement restricted through a pass system as well as outright violence against Indigenous people. Following the Gold Rush, this oppression was worsened as mass murder by both individuals and vigilante groups killed thousands of Native people and resistance often failed against the massive wave of prospectors and settlers, creating what is now referred to as the California Genocide.