Via Starwars.com:

THE GALAXY’S WHITE-ARMORED SOLDIERS GREW MORE RUTHLESS AND FACELESS AS REGIMES CHANGED.
A military filled with well-trained soldiers in white armor is a common trait shared by the Galactic Republic, Galactic Empire, and First Order. This is no coincidence, as each generation of combat forces influenced the battle dress of its successor. However, the Republic’s clone troopers, the Empire’s stormtroopers, and the First Order’s stormtroopers have their own unique attributes, with various levels of disparity existing in their recruitment processes, training procedures, levels of conformity, and the images they project to citizens living in the territories that they control.

While all three entities filled their ranks with humans, the Republic obviously did so with human clones who had their aging accelerated and were trained from birth on Kamino. While the initial army of Imperial stormtroopers was originally comprised of former clone troopers, the Empire gradually phased in human recruits who eventually became the core of its fighting force. The First Order’s enlistment tactics were similar to the Republic’s breeding programs in that they were morally questionable, except the First Order resorted to identifying potential soldiers and stealing them from their families at a very young age. By default, the oppressive Imperial regime ironically had the fairest program with its use of voluntary recruits.

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