California has twice the economic power of Texas or New York and hell of a lot more people...not mention a couple naval, army, and air force bases.
It is well protected from incursion by a large moutain range and deserts (sorry Nevada) except to north, which is Oregon (a state which would pose no threat).
Most important industries are well represented in California and CA would control commerical ports for products from the Far East.
Aside from Texas, New York, and Florida (and maybe one or two other states) which also lose money in the federal taxation / re-allocation process...all other states would stop receiving their yearly federal budget handout that CA, TX, NY, and FL pay for and these states would find themselves in a very real budget crisis.
Basically, CA, TX, NY, and FL would sign a treaty... all other states would be fvcked.