text: california used to belong to  mexico?   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.