California Labor Code 2922 provides that unless an employment contract has a specified termination date at least one month later than the date upon which employment is to commence, that the employment relationship is "at will," which means that the employer can terminate the employee, at any time, for any reason, or for no reason at all.
There are numerous exceptions -- including discrimination and breach of contract claims.
Your facts don't suggest any unlawful discrimination in the hiring process (race, color, nationality, ancestry, religion, sex, sexual orientation, pregnancy, unequal pay between sexes, age and disability). However, since California's discrimination laws are more comprehensive than federal or Illinios, I have provided the list, in case something jumps out at you that could affect your thought process.
Assuming no discrimination, then the question is whether or not you relied to your detriment on anything in the offer letter. In other words, did the employer make a promise which caused you to expend resources in reliance on that promise, such that the revocation of the offer causes you damage?
If yes, then you could sue for your damages -- but, nothing else. Whereas, if you have a discrimination claim, you could sue for the value of the job itself.
Those are the two options that appear to fit your circumstances. If you think there's discrimination involved, then you can complain to the Department of Fair Employment and Housing.
For damages associated with a breach of contract, if they amount at stake is $7,500 or less, you could sue in small claims
-- but, you would have to come to California to do it.
Hope this helps.
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