While many people are unaware of its existence, the employment-at-will doctrine allows employers to do exactly what the title implies; hire and fire employees at will, with a few limited exceptions. It is incredible that a workplace legal principle from the Reconstruction era has survived basically unscathed into the 21st century, with little modification.
The doctrine was devised in 1877 by Horace Gray Wood. Incredibly, as an intellectual basis of the doctrine, Wood analyzed master and slave relationships in formulating a lasting policy for arbitrating employer and employee relationships. Obviously, this is not how we should be regulating labor relations in the 21st Century.