The Star Wars saga’s Galactic Empire and First Order both use stormtroopers but there is a major difference between the ground forces of these two factions. The Galactic Empire has both a regular army and the Stormtrooper Corps, while the First Order’s army is exclusively its Stormtrooper Corps. For practical and ideological reasons, as well as the Star Wars sequel trilogy’s meta-narrative, the First Order’s lack of a regular army makes sense.
In the Legends Star Wars continuity, stormtroopers are not the primary ground forces of the Empire, with the Stormtrooper Corps being a separate branch of the Imperial military that answers directly to Palpatine himself and works alongside the Imperial Army and Navy. While the stormtroopers were deployed to fight the Empire’s most important battles, the more common and less formidable Imperial Army Troopers fought on less strategically important worlds. In the post-2014 Star Wars canon continuity, stormtroopers are still elite soldiers compared to Imperial Army Troopers, but the Stormtrooper Corps is a sub-division within the Imperial Army.
Although the First Order is the Empire’s successor state in the canon timeline, the First Order’s military is not identical to that of the Galactic Empire. The First Order’s stormtroopers are their only ground troops, which fits with the First Order’s stronger emphasis on quality than quantity, especially when compared to the canon version of the Empire. Stormtroopers are also far more iconic both in-universe and in the real world, so the exclusive use of stormtroopers as the First Order’s ground forces is a case of the Star Wars narrative and meta-narrative aligning perfectly.
The Galactic Republic popularized elite white-armored troops in-universe via the clone troopers, who comprised the bulk of the Republic’s military. Before the Clone Army, the Republic only had the limited Judicial Forces to protect its worlds, and the Clone Army itself is a small and superlatively effective force meant to back up the Jedi Knights and hold back the overwhelming numbers of the Separatists. When Palpatine’s fascist coup wiped out the Jedi and putrefied the Republic into the Empire, Judicial Forces and non-clone planetary defense forces would form the Imperial Army’s soldiers, while the best-recruited beings replaced the elite clone troopers with Imperial Stormtroopers.
The First Order, while vast and powerful, is far smaller than the Empire it idolizes, so relying exclusively on an elite force of stormtroopers is a necessity. Unlike the Empire, which treated its most well-trained stormtroopers and TIE Fighter pilots as disposable, the First Order needed to preserve its personnel in its war against the Resistance, so its basic TIE Fighters came equipped with hyperdrives and deflector shields and its army only consisted of elite stormtroopers. The First Order’s use of stormtroopers served more than a practical purpose, however.
The First Order, as a fanatical fascist junta, was obsessed with replicating the glory days of the Empire and focused heavily on pageantry imbuing classic Imperial armor and starfighter styles with modern technology. Most galactic citizens are far more familiar with stormtroopers than Imperial Army Troopers, so the First Order only brings back the Empire’s most famous type of ground troop. From a real-world perspective, most viewers aren’t familiar with Imperial Army Troopers, but stormtroopers are pop culture icons, so the Star Wars sequel trilogy, which consciously tried to replicate the look and feel of the original trilogy, only used a new iteration of the most famous Imperial soldiers for the First Order’s army.
