"We are an Empire ruled by the majority! An Empire ruled by a new Constitution! An Empire of laws, not of politicians! An Empire devoted to the preservation of a just society. Of a safe and secure society! We are an Empire that will stand for ten thousand years!"

The Galactic Empire's History.

As the Clone Wars reached its end, the clone trooper veterans that defended the Old Republic emerged as the feared enforcers of the New Order. When Emperor Palpatine declared the formation of the first Galactic Empire, the clones of the late Jango Fett were rechristened as Imperial stormtroopers. Without the guidance of their Jedi officers, stormtroopers were "cut loose" to brutally enforce the new Imperial doctrine, to which they quickly built a reputation as merciless, uncompromising soldiers indoctrinated with unconditional loyalty to Emperor Palpatine.
Stormtroopers were the elite soldiers of the Galactic Empire. Like Imperial-class Star Destroyers and TIE Fighters, stormtroopers served as ever present reminders of the Emperor's power, extensions of his will, and a method of keeping the Empire's thousands of star systems in line through fear. With few exceptions, the stormtroopers were distinguished from all other Imperial military units by their signature white armor.
The Imperial stormtroopers originated from the clone troopers of the Grand Army of the Republic. When the Galactic Republic was reorganized into the first Galactic Empire, the Grand Army was reformed into the Stormtrooper Corps. Under the Empire, stormtroopers operated alongside Imperial Army and Navy units independently, while some placed on Naval ships were given the name "marines."