[p4ul]

In the US, most employers roughly follow the guidelines that the Federal government uses [1]. Which is to say, that employment is "at will", and either the employer or employee can terminate the relationship at any time. The obvious caveat is that someone cannot be fired for a reason that would be illegal (e.g., firing someone because of age, gender, religion, etc.).

In practice, however, most organizations are extremely risk-averse when it comes to wrongful termination lawsuits. And as such, they'll have a lengthy "performance-improvement plan" (PIP) that can last three to six months and is meant to serve as evidence that an employee's poor performance was the reason for dismissal.

There are also, of course, extreme cases where someone can be fired "for cause" when they've committed some kind of egregious violation (e.g., embezzling funds, fraud, corporate espionage).

[1.] https://www.usa.gov/termination-for-employers