Noncompetition agreements are, with some narrow exceptions, unenforceable in California. If your contract says that the law of another state will apply, then you may have issues if you try to compete. So many factors are involved that it is best to cross that bridge when you get to it.
You can always be sued and, if so, it would then be up to you to defend yourself. This is true whether or not non-competition clauses are enforceable as it's always "up to the judge" to decide the legality of any such provision. I agree with my colleague that, with limited exception, non-competition clauses are not enforceable in CA.
However, few cases allege only a violation of a non-competition agreement. They also frequently allege unfair trade practices and theft of trade secrets. Neither you, nor your employer, wishes to be caught up in litigation following your employment. If I were you I would, at a minimum, consult an attorney and discuss your situation as you do not want to find yourself out of a job.
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The answer to your last question is yes, your employer can sue you. Will he/she prevail is a different matter.
That being said, I agree with the other attorneys that non-compete and non-solicitation agreements are generally unenforceable in CA. That should give you a good idea of what your legal rights are.
The bottom line is that there's no use worrying about what your employer will do or not do until it happens.
I agree with what's been said with respect to anybody can sue anybody, and non-competes are generally unenforceable in California. However, that "generally" doesn't mean always, and you could be on the receiving end of a serious lawsuit and injunction filed in another state depending on how your agreement is drafted. It really does happen, it is not just a theorectical occurence. You'll need to evalaute your risks by sitting down with an employment attorney, an qucik & easy answer to a non-compete question can mislead you.
THIS IS A GENERAL ANSWER TO A GENERAL QUESTION AND SHOULD NOT BE RELIED UPON AS A FULL LEGAL ANALYSIS OF ANY FACTUAL MATTER. An attorney-client relationship is not established or offered solely as a result of this answer.