CA has the advantage of geography - any invading army would have to cross the steep side of the Sierras. These have relatively few passes, most at high altitude and all easily defended against an invading army. It's actually easier to invade CA via Tijuana than overland through the U.S.
They also have the advantage of being exceptionally technically advanced - perhaps moreso than the U.S. military, given that Lockheed has a large presence in the Bay Area and all the Predator drones are made in San Diego. And they're a nuclear power - Livermore is one of two sites that actually develops atomic bombs.
This'd be fun to wargame out as a board game or computer simulation, although the subject matter is a little macabre.
If this little wargame is to be played it would likely be a battle of attrition US warships deploy to restrain trade and cut off the CA economy... assuming the leadership isn't removed somehow by assassination or coup attempts.
CA is a big part of the US economy and culture, but in no way would I imagine that it could resist the rest of the 49 states.
You have to consider that it's highly unlikely that everyone in CA would be in favor of secession. I can imagine that many people in the Central Valley, who tend to be far more conservative than their coastal counterparts and rely extensively on Federal farm subsidies as well as out of state water resources would be less than thrilled at the idea. These people also tend to have high rates of firearms ownership so I could see a nice little insurgency springing up.
Certainly there is a lot of high tech industry in CA but I doubt that Lockheed and the other defense corps are disproportionately reliant on CA, they have plants and R&D centers spread throughout the country.
It actually might now. The Right is in full control of the country, and it had been generally very sympathetic to the idea of just splitting up the country and each going their own way lately. It was never really seriously looked at by the Left - sure, there are semi-joking proposals like Cascadia, but nothing as serious as, say, Texas secessionists. But right now Cascadia might sound like a very good idea for a lot of people.
The better more workable suggestion for these times is for the Federal Government to give the states back more of their individual powers. The Republican party is certainly more open to that vs. the Democrats.
It would have been workable, if not for one thing that Republicans have consistently promised their electorate for a long time now: a federal abortion ban. Liberals will not see anything including that as a legitimate "states rights" solution for themselves. Secession resolves this issue, and can even score points with the electorate on the right.
Do you think the US would let California leave? Can CA defend her own borders (especially against the rest of the US)?