If California were to secede what would stop us from simply rolling in with the army and... idk, taking it back?
It would operate the same way the Civil War did. From the perspective of the federal government, states
cannot secede. Lincoln's position was that secession was unconstitutional, so none of the states actually left. After the Civil War, the Supreme Court officially made it unconstitutional.
So yeah, they'd roll in and kick the rebels out.