According to the Lore book (which I referenced for my entire post) the Beastmen started summoning their Primals right after the Battle for Silvertear sky, where Midgardsormr and the Garlean fleet fought.
To quote the relevant section:
"Not long after Midgardsormr's demise, something unexplained occurred. Eorzea's beast tribes began summoning their own gods - the "primals" - into the corporeal realm. Dubbed "eikons" by the Garleans who questioned their divine origins, these powerful creatures were deemed a threat to imperial rule and soon made a target of its armies. XIVth Legion legatus, Gaius van Baelsar, quickly observed the futility of engaging the eikons in battle, as the vanquished "gods" were brought back time and again through the use of crystals and prayer. Moreover, there were increasing reports that the "blessings" of these primal beings were transforming men into fanatics who would turn on their own kind in the names of their new rulers. Without a proper military strategy to address the eikon threat, Emperor Solus zos Galvus would eventually order Baelsar to suspend the Eorzean campaign." Page 38, Encyclopaedia Eorzea.
While the failed first invasion did force them to pull back momentarily, it was the rise of the primal threat that caused Garlemald to call of any further invasion attempts, at least until they could find a solution to the primal problem.
Nael came to the attention of the emperor with his successful conquests in Orthad. As the emperor was looking for a why to deal with the primals and annex Eorzea Nael came to him with a potential solution. In his families archive of Allag records they had found evidence that Dalamud was actually a Allag satellite of extraordinary power. The Garleans first attempts to contact Dalamud resulted in the Bozja Citidel where the lunar transmitter was situated and the city around it being vaporised, along with Cid's dad. This put the Emperor of the idea but Nael convinced the emperor to reconsider by suggesting drawing this power down on Eorzea to resolve the stalemate there. Thus Nael, with the emperor's blessing, instigated the Meteor project.
Relevant quote:
"Neal van Darnus, however, was not yet ready to abandon his obsession with the Meteor Project, refusing to allow the efforts of House Darnus to be left for naught. After learning of his deployment to Eorzea, van Darnus knew his wait was over and seized upon the opportunity to appeal to the Emperor regarding the merits of resurrecting the project, claiming he had discovered a means by which to successfully control Dalamud. After enduring a decade-long stalemate on the Eorzean front, the aged Emperor, impatient to solidify his legacy before his death, had once again become willing to entertain the notion of a realm-wide "cleansing' of the beastmen and their eikons. Thus the second phase of the Meteor Project was commenced." Page 41, Encyclopaedia Eorzea
In short that means what happened in 1.0 was a mission by the Garlean Empire to do pretty much exactly what Nael set out to do, pull down Dalamud, with authorisation by the Emperor. Gaius didn't like the idea as he wanted there to be something to conquer but since the orders were from the Emperor he didn't have much choice. Every Garlean incursion in 1.0 wasn't part of an invasion but to set up what was required to carry out the Meteor project. The lore book also states it was mostly the 7th Legion that was involved in incursions in Eorzea. The 14th mostly stayed in support in Ala Mhigo.
MORE TO COME.
