The Clone Wars was over, the Jedi threat was eliminated and the few remaining Separatist holdouts were being dispatched. Out of all this, the clone trooper veterans emerged as the newly christened Imperial Stormtroopers: defenders of the Emperor's New Order. Without the guidance of the Jedi, the Stormtroopers became known as being brutally efficient, even merciless. Initially, the Stormtrooper Corps was composed entirely of clones off of the Fett template. During this time, traces of the obsolete Phase II armor remained in the design of the improved armor. After the Battle of Kamino, also known as the Kamino Uprising, in 12 BBY, Fett clones ceased to dominate the Stormtrooper ranks.
By the time the Galactic Civil War truly began, Fett clones had been almost completely supplanted by other clones from a variety of different templates. After 9 BBY, Humans began to enlist in the Stormtrooper Corps. The Fett clones never fully came to terms with serving alongside other clones and recruits, feeling that they were inferior (which was probably true). By 0 BBY, Fett clones composed 1/3 of the total Stormtrooper ranks, and the number of Human conscripts was growing. No aliens and very few women were accepted into the Corps.
For twenty-three years, the Stormtroopers suffered very few defeats until the catastrophic Battle of Endor, where an entire legion of the Emperor's best troops were defeated by an Ewok tribe. The disastrous outcome of the battle cost the Empire its "unbreakable" grip on the galaxy. In addition, news of the Empire's finest being defeated by a group of primitives dealt a severe blow to the Stormtrooper's reputation and numerous planetary revolts followed. The many efforts to turn the war back in he Empire's favor ultimately failed. Throughout the next 19 years, the Stormtroopers suffered a series of defeats that left their ranks diminished and their morale crushed. By 19 ABY, the shattered remains of the Empire (now the Imperial Remnant) finally conceded defeat and signed a treaty. The Stormtrooper Corps survived, now a broken shadow of what it once was, relegated to guard and police duties on the remaining Imperial worlds.
Several other Imperial off-shoots also used Stormtroopers, notably the Pentastar Alignment and the Empire of the Hand. Over the years, the Imperial Stormtrooper Corps would slowly rebuild, relaxing its anti-alien policies to increase its ranks. By the time of the Yuuzhan Vong War, the Stormtroopers would once again be known as an elite fighting force, seeing combat alongside its former enemies (the New Republic, later the Galactic Alliance). As the Imperial Remnant began to rebuild, it could no longer rely on clones to fill the ranks. However, most of the Emperor's New Order doctrines also had died, being replaced with a less totalitarian government. This allowed the Remnant to call on something it never could as the Empire: patriotism. Ultimately the gamble to use a brutal training regimen for recruits, rather than relying on preprogrammed clones, payed off. By 40 ABY, the Remnant's Stormtroopers were equal in marksmanship to the Empire's, and the shocktroopers were said to be as effective as Vader's 501st Legion. Stormtroopers would continue to see service through the Second Imperial Civil War, beginning in 130 ABY. They were instrumental in winning the war and establishing the new Galactic Federation Triumvirate, finally unifying the galaxy.
Stormtroopers are the tier 2 infantry of most of the Imperial factions. Unlike the basic Army Troopers and Pentastar Enforcers, Stormtroopers can fire while on the move which gives them a significant advantage while attacking a position.
The infantry rush is still one of the most potent tactics available to any faction. In short, the Stormtrooper is one of the most important unit to the Imperial factions and can be used in any situation against any resistance (In the right numbers of course).