Can I be fired without any warning?

At-will cuts both ways — you can quit without notice, and the employer can terminate without notice. No federal law requires warnings, progressive discipline, or a reason. What changes the answer:

Protected reasons. A termination motivated by a protected characteristic under federal EEO laws, or by retaliation for a discrimination complaint, wage discussion, FMLA request, or workers' comp claim, is unlawful no matter how it's labeled. Timing is evidence: a sudden firing days after a protected complaint invites scrutiny even when a valid reason exists.

Promised procedures. If an employment contract, union agreement, or even handbook language promises warnings before termination, skipping them can be a breach — this is why employers write at-will disclaimers, and why the promises that do exist are enforceable.

Montana. After the probationary period, Montana's Wrongful Discharge from Employment Act requires good cause — the one state where "no warning, no reason" doesn't hold.