The significance of the Mexican-American War, which took place from 1846 to 1848, lies in its lasting impact on U.S. territorial expansion and relations with Mexico. The conflict was primarily fueled by the U.S. desire to acquire territory in the West, including Texas, California, and New Mexico. The war ended with the Treaty of Guadalupe Hidalgo, resulting in the U.S. gaining over 500,000 square miles of land. This expansion intensified debates over slavery in new territories, contributed to sectional tensions, and set the stage for future conflicts leading to the Civil War. It also defined the U.S.-Mexico border.