Not if you signed a contract declaring that you will show up.
So a strike is in one way organised contract breaching - but one that is allowed under certain circumstances under the bigger contract, the state laws (which you accept by being in that state).
Yes, even then. A contract violation is not a violation of law. My question is about it being illegal.
Breach of contract is a violation of civil (but not criminal) law. It's illegal but it's not a crime. The question is more of enforceability and of compensation due, if any.
False; contract law is, as the name suggests, law, and there are legally established remedies for violations.
(Speaking about how things are in the US. I don't know how they are elsewhere.)
Contract law is about how to interpret and adjudicate contracts when there is a dispute between the parties in the contract.
Nothing about that means that violating a contract, in and of itself, is illegal. You aren't going to suffer any criminal penalties for failing to adhere to your contractual obligations.
What you may do is suffer whatever penalties that the contract specifies will happen if the contract is violated, and the legal system is also geared to work out and enforce such things. But that's a world apart from the notion that violating a contract is "illegal".
Its a violation of legal rights for which there are legal remedies. Claiming that its not a violation of law (the original upthread claim) is... well, as wrong as it is for any other violation of civil law. (“illegal" adds more complications because “illegal” has multiple overlapping senses, it can apply to any violation of law, or it can be used to refer specifically to criminal violations.)
> What you may do is suffer whatever penalties that the contract specifies will happen if the contract is violated
No, contract law itself specifies penalties for contract violations. Contracts may attempt to specify damages for certain violations as a replacement for the usual calculation of actual damages (“liquidated damages” provisions), but contract law governs when these will be given effect instead of the usual assessment of actual damages. It is neither necessary for there to be a specified penalty in the contract for one to be applied, nor will a specified penalty automatically be applied.
When we are talking about the law, of course one has to be pedantic. Apart from that, the sibling posters expressed my reply very well already.