Wrong. Stolen from who? At the time, California was sparsely populated by the Spanish, and for a short time by Mexicans (as well as Russians in the northern half). California was bought from Mexico after the war - that's not theft, that was a purchase. We even forgave Mexican debts to the USA and their citizens. At the time, the California purchase included an area encompassing several western states. If Mexico wants it, they should offer payment at today's prices, with interest.
That would be like the Russians pouring into Alaska because we “stole” Alaska from them.