The damage is not what the Californians inflict on the US military, it is the damage the leftist media will inflict on Trump while he is quelling the insurrection.
I suspect it is what the left wants, they know California will stay in the US but they want bodies to lay at Trump’s feet with the goal of delegitimizing him, which makes the threat of them leaving greater.
This is truly an existential threat and should be taken seriously.
I think this Nation is headed for a Civil War, like it or not. Leftists are very dangerous. History is full of bloody wars that end in communist rule. It would be prudent to be making plans and alliances.