First Wasteland-Eidel War

Eidel's conquest of the forested regions surrounding their home went relatively smoothly. Pushing south into the Wastelands, however, they found a new sort of opponent entirely.

Eideli Forces

Spindles (or lack thereof)

This war took place before the standardization of the Eideli legion structure, at a time where the Eideli military relied much more heavily on manpower. It was during and after this conflict that spindles grew in prominence as foot soldiers, a direct consequence of facing Wastelander shamans capable of turning any corpse on the battlefield into another fighter (if not several) for their side. A spindle could not be raised in the same manner, and the leech-effect of spindles would hamper (though not outright prevent) the shamanic rituals.
While the Eideli legions would not be so refined in organization for another few centuries, it was this conflict that pushed Eidel towards an overwhelmingly spindle-based military. To be precise, it was this conflict that pushed the spindle production and maintenance process to be streamlined enough as to effectively create soldiers. Part of this standardization included the development of the spirit jar, used to transport shadow spirits instead of forcing one to draw directly from Naomhsgire's ghost storm, allowing spindle production to take place throughout the empire. Visual differences were also apparent: the jars' effect of washing out coloration resulted in all such spindles having the same uniform white glow as opposed to the rainbow mishmash previously exhibited.
The development of the spirit jar was a massive boon in the long run, but did little to help during the war itself: while more spindles could be created, the specific infrastructure to support them (notably in balancing energy levels to avoid rampancy and suboptimal performance), while significantly more efficient than humans', could not also be rolled out in time. Note that another large part of the shift to spindles was that, well, Eidel lost. Their military forces were significantly depleted, and spindles were used to replenish their numbers.

Tactics

A second key factor in the Wastelander repulsion of Eideli forces was the nature of their civilization: Eidel was extremely used to fighting against rooted civilizations, where taking the capital city etc. essentially spelled victory. The Wastelanders, on the other hand, were largely nomadic, only having a few stable settlements far within the heartlands. Eidel's initial plans involved marching their army to take these known capitals, but found themselves constantly harassed by raiding parties. After being worn down, more organized forces of similar numbers to their own would meet them on the battlefield.
This is before combat alchemist units were a thing: xiphaem as a practice was essentially non-existent in Eidel at the time. Wasteland raiding parties were so successful because they could use xiphaem reinforcement to dramatically outpace the running speed of Eideli soldiers. It was veterans' tales of what Wastelanders could do which captured the imaginations of alchemists throughout the empire, who realized that much of the described phenomena functioned within alchemic principles. As a result, the Eideli practice of xiphaem was born (this is also why combat alchemist units are named as such, despite nominally using xiphaem). Only the existence of arcahe riders stopped the Eideli from being entirely picked apart.

Aftermath

Despite losing, this war is often looked upon fondly by Eideli historians as 'The War of a Thousand Lessons'. A significant part of what made Eidel the superpower it became within less than a hundred years came out of lessons learned during this war. Things would have been very different for Eidel had the Wastelands not devolved into civil war practically the moment that they repelled their invaders, as the combined Wastelander force could easily have swept through the fledgling empire.