The effects of Manifest Destiny were massive, especially with territorial gains. Few nations had ever expanded so quickly. The US annexed Texas after an revolution, secured the Oregon Country in a treaty with England, and gained all of California (and more) as a result of the Mexican War. The territorial expansion was remarkable, providing the growing US with farmland, waterways, resources, and access to the Pacific for travel and trade. An "American" culture spread across the continent as pioneers moved westward, bringing civilization to a formerly uncivilized place.
FIND OUT MORE ABOUT THE MEXICAN WAR HERE!
However, not all of the results were positive for everyone involved. The native population of the West saw Manifest Destiny as a beginning of the end. As settlers moved out to Native American lands in future decades, resistance was short lived and the Indian population would either be slaughtered in battle or moved to reservations, losing their ancestral lands and way of life. The relationship between the United Sates and Mexico would be strained for decades over the legitimacy of the Mexican War. Most importantly, the expansion led to debates about the fate of slavery in the West. Should the institution of slavery spread to the West and keep the balance of free and slave states in Congress? This question served as the crucial dividing wedge between the North and South, ultimately leading to the collapse of national unity and a bloody four-year civil war.