If Europeans should go back to Europe then it follows that Mexicans, who are as "European" as anyone else, should go back to Spain. Mexicans have no more of a "right" to California than Americans do.
Am I living in some weird alternate reality where it is OK for Spanish nationals to conquer land, but White Europeans are not allowed? Does the fact that Spanish Europeans skin is slightly darker than other European skin give them some kind of 'special license'?
Europeans are Europeans. They were all doing the same stuff and were competing against each other. The only difference between Spanish Europeans and White Europeans, is that the White Europeans were smarter, tougher, and stronger, and kicked the ass of the Spanish. And now they want what they call 'their land' back?
Then why not give the Louisiana Purchase back to France?
This is all distorted history and insanity.