This is in the news again, in San Bernardino County—a county next to L.A., which is home to over 2 million people (AP):
The county's Board of Supervisors voted 4-0 on Wednesday to put the secession measure on the 2022 ballot, the Southern California News Group reported. One supervisor was absent.
The measure will go before the board again next Tuesday for final adoption. The initial draft would put this question to voters on Nov. 8: "Do the citizens of San Bernardino County want the San Bernardino County Board of Supervisors to study all options to obtain its fair share of state and federal resources, up to and including secession?"
Can they do that? Why, yes, they can, but only with permission of the state from which they're seceding as well as of the federal government. Article IV, section 3 of the U.S. Constitution provides,