text: Yes, acctually the states that we know as California, Nevada, Arizona, New Mexico, and Utah were all once a part of what was known as Northern Mexico.  However, due to a treaty signed on February 2, 1848, known as, The Treaty of Guadeloupe Hidalgo, the U.S. annexed the Northern half of Mexico to a tune of 15 million dollars.  This all came at the conclusion of the Mexican - American War, which started due to the Westward expansion of Americans and the Texas war for Independence.