At-will, and what it really means
Absent a contract saying otherwise, either side can end the employment at any time for any reason or no reason. Being fired unfairly, abruptly or for something trivial is generally lawful.
That is the default. The whole of wrongful termination law consists of the exceptions to it.
The exceptions that matter
Termination because of a protected characteristic. Termination in retaliation for asserting a legal right — reporting harassment, filing a wage claim, taking protected leave, refusing to do something unlawful. Termination that breaches an actual contract. And termination that violates a fundamental public policy.
Retaliation claims are the most common of these in practice, and often the strongest, because the timing tends to be documented by the employer's own records.
Why timing is the central fact
Almost every retaliation case turns on sequence: you did the protected thing, and then something changed. Shifts cut, performance reviews that suddenly diverge from three years of history, a role restructured.
This is why writing down dates matters more than characterising motives. A clean timeline is persuasive; an argument about what someone was thinking is not.
Preserve the record before you lose access
Once employment ends, so does access to email, systems and documents. Forward nothing confidential, but do keep copies of your own performance reviews, offer letter, handbook and any correspondence about the issue you raised.