The Mexican-American War was a conflict between the United States and Mexico that took place from 1846 to 1848. The war started because of a dispute over the Texas border. The United States claimed that the border was at the Rio Grande, while Mexico believed it was at the Nueces River.
The war ended with the Treaty of Guadalupe Hidalgo, which gave the United States a large portion of what is now the American Southwest, including California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado, Wyoming, Kansas, and Oklahoma. The war was controversial at the time and is still debated today, but it played a significant role in shaping the borders and identities of both the United States and Mexico.