The vaunted "Mexican" posession of California was a joke: at the time of the Mexican-American war, there were approximately 7000 men, women and children who held loyalty to Mexico in California. By contrast, there were 130,000 aboriginals in southern California alone. Of course, that was about 100,000 fewer than had originally been there - they died off because of disease and oppression by the Spaniards (later "Mexicans" after the 1821 finish of the first Mexican revolution). So who exactly has their roots there? If you are a Yuman (Kameyaay) tribesmen, then you have a claim. Which is, of course, why they have reservations (the Kameyaay are mostly well to do now since they own casinos...).
In 1848 there were 30,000 Americans in the upper Sacramento river valley, and by 1850, over 250,000. The "Mexicans" - remnants of a failed attempt by the King of Spain to assert his territorial rights granted him by the Pope in the Treaty of Tordesillas - were outnumbered 10 to 1 by Americans, and by a similar number of aboriginal tribes. So, umm, just where was this mythical Aztlan? Answer: NOWHERE but in the minds of a resentful group that has failed over and over throughout their awful 500 year history.
By the way - currently the Tohono O'odham tribe in Arizona is pretty upset about illegals coming across their land. This has confused many American leftists, thinking that they would immediately join up with their Mexican "brothers". This is laughable, because one of the major reasons that reservations tend to be on this side of the border, was that no matter how bad the Americans were, they were infinitely better than life under the depraved Mexican feudal hierarchy. Mexican massacres of aboriginal tribes are well known and documented, and the Apaches, Pima's, Navajo and Tohono O'odham ("papago", a Mexican name they hated) all remember that.
What is your source?