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The only bit of information I can find even close to that is the Sightseeing Log entry for Ala Mhigo, which states that it was built in 6 AE 1335 to "repel invaders from Ilsabard." This suggests Ilsabardian nations, pre-Empire, weren't civil or were nonexistent and southern (or at least southwestern) Ilsabard was populated by nomad tribes and the like. The Garlean ancestral homelands are also temperate in comparison to the harsh northlands; beyond that, no information is available.
Given Ala Mhigo was built in 6 AE 1335 and the Garleans were pushed to northern Ilsabard ca. 6 AE 800, it's very unlikely the Eorzean nations were responsible. The few Othardian nations cannot be accounted for, but given how far removed they are from southern Ilsabard the likelihood of guilt on their part is also low. Ilsabard's geography remains unknown.
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The lorebook says that the tribes in Ala Mhigo were unified in around 1100 of the 6th AE so about 200 years after the Garleans became a unified people. The Ala Mhigans also had a steady number of attempts at invasion from the north as it was the only land route into the more wealthy inner reaches of Eorzea. Ala Mhigo was build to keep out invaders. The Ala Mhigans did build up into Islibard but only as a watchtower system along the major trade route to the east to protect traders traveling the route.
Honestly the nations that caused the Garleans so much strife during its existence sound like they were among the first to fall. Most of those countries would be almost fully assimilated at this point. Only the oldest of natives would remember life before Garlean rule.