Shed some Garlean light?

[Enkidoh]

Dikatis is totally correct - Garlemald existed as a republic for centuries prior to it becoming the Empire due to early on learning to harness the vast reserves of aether-rich cereleum found in the frozen wastes of the place, initially as a heating source (it burns extremely hot with a bright blue flame, rather like natural gas), as it was the only way they were able to live in such a frigid environment.

Solus came along much later, and whom at some point was possessed by Emet-selch (probably about when 'Solus' was mentioned as first beginning his rise as just a lowly engineer with introducing magitek technologies, starting with the cereleum engine.).

At that point the actual person who was Solus was effectively dead and gone - his will and personality erased by Emet's dominant Unsundered soul, and from that point it was all Emet wearing Solus's face as a meatsuit (and he continued to do so long after 'Solus''s mortal body expired - using Allagan cloning technology to make himself essentially a wardrobe of "Young Solus meatsuits" (which became something of his iconic look, ironically).

And Emet certainly had a hand in the formation of other empires throughout history including Allag - he outright and very gleefuly boasted of that fact when he first introduced himself to the Warrior of Light and the Scions in the Crystarium during Shadowbringers (which at the time he was in the process of creating yet another pseudo-empire on the First in the form of Eulmore - he was the one responsible for showing Eulmore how to control the sineaters by having Eulmore's governor infuse his then unborn son with sineater essence, turning him into a Lightwarden, as part of his plan to rejoin the First) - his empire building naturally coming to an abrupt end though under the ocean when you punch an axe sized hole through his chest though.