An at-will employee can be fired at any time, for any reason (with a few illegal exceptions). The employer does not need good cause to fire you if you are employed at will.
Montana is the only state which protects employees from “at will” policies, and that only after the employee has completed an additional “probationary period”.
Note that the law will generally presume you are employed at will unless you can prove otherwise. For this reason it is very important to keep copies of any documents relating to your employment which you have signed.