Mexico expelled Spain about 1821 and then claimed Spain's American empire was theirs.
Mexico only had possession of California for about 30 years. And Spain's claim to the land was flimsy to begin with. Oh yes, and how sad for the Indians of California who had been stealing from each other.
I have heard that the idea of “laid-back Californians” goes all the way back to pre-Columbian times in that local tribes didn’t fight amongst themselves as much as in other areas of North America.
Spain’s settlement of California was well-documented; Lewis & Clark knew they had to avoid Spanish interference when they surveyed the Louisiana Purchase because it abutted Spain’s recognized claims at the time. Otherwise the cities would be named “The Angels”, “St. Francis”, and “Sacrament”...