Dismissing an employee is always a difficult task, and it can negatively affect both the employee and the employer—especially if it is mishandled. Most employers will try to avoid dismissal and will employ progressive discipline (warnings and other disciplinary acts) in an attempt to improve the situation.
In the United States, the doctrine of employment-at-will allows employers to fire employees without giving a reason (as long as the reason isn't illegal, such as discrimination based on age, gender, or race). Most companies dismiss a worker for a specific reason.