A new bid to place a measure on the ballot that would ask California voters to approve the state's secession from the U.S. has been cleared for signature gathering.
New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
If a state agrees, a new state can be formed in its territory, effectively splitting it.
Article IV Section 3 of the US constitution
If a state agrees, a new state can be formed in its territory, effectively splitting it.