Almost half of all Mexican territory was given to the United States as a result of the Mexican-American War. The Mexican-American War officially ended with the Treaty of Guadalupe Hidalgo. In this treaty, Mexico agreed to giving up a significant portion of their land (known as the Mexican Cession). This portion of land contained many states that the US has today including: California, Arizona, New Mexico, Utah, and Colorado.
Essentially, anything west of the Rio Grande became part of the United States.