Most US Corporations will have an "at will" clause in every signed employment contract.    My current company does - as did previous U.S. companies I have worked for.   In Canada, even if you work for a U.S. company and signed an "at will" contract - the "at will" clause is not enforceable here and Canadian standards for notice apply.

But in the U.S., the "At Will" essentially means that you can be fired for any reason at any time.    No breach of contract unless you were told (hopefully in writing) that you were not employed "At will".     

In this case, Google would probably pay Damore out vs having this drag on in court, even though Google has the upper hand here - both legally and from a resources point of view.    That's Damore's best hope.