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.
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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
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