California was part of Mexico for only about 25 years, and even then, without the permission or approval of Californians. The last 15 years of those 25, there were a series of revolts against Mexican rule. The conquista failed, and there’s no justice in the calls for a “reconquista”.
The Mexicans made ludicrous claims to territories (lots more than California) they were unable to rule. The Indians, Apache and Comanche had the run of these lands Mexico said were theirs. In fact even after the 1848 war, under the US government, the Indians ran wild. It took the US Army in the 1870s to subdue these wild Injuns who I have respect for.