Again there is this victim-blaming going on that is trying to paint Eorzea in a negative light simply because we rose up and successfully drove the Garleans out. What was the Alliance meant to do? Sit there while Ilberd taunted the Garleans with a giant Primal? Stick their heads in the sand and pretend "everything will be alright?" Of course not! We couldn't spend all our time sitting on our hands and just waiting to defend against Garlean attacks, especially as they were being led on a string by the Ascians who were just conveniently dropping things like the Ultima Weapon into their laps. We had no choice but to seize the initiative and take the fight to them.

And if you want to be truly technical about the war, really, the war never really ended after Gaius fled Lake Silvertear with his tail between his legs. The Garleans did not stop fighting Eorzea due to a peace treaty or cease-fire, they simply drew back because they encountered at Silvertear a force that was too much for them to handle, while still remaining in a state of war. A cold war, true, but a war none the less, and a cold war that occasionally flared hot only to smoulder away again, as the Garleans continued to make small-scale military operations into Eorzea, attacking aetheryte camps and the like with impunity. To say nothing of the whole Dalamud affair and the Calamity, which was fully endorsed by the Emperor.

And after, Gaius moved and seized land and built strongholds, again with impunity. The only reason the Alliance did nothing is they were powerless to do so. So this shows that the war was still on, it was just smouldering rather than blazing hot until Operation Archon where we took the fight to them (and all the Alliance really did was blockade, not capture, the Garlean strongholds, the Praetorium's destruction was all Lahabrea's doing).

Thus, Baelsar's War and everything that came after was simply an extension of the cold war that has smouldered and occasionally flared up since the Garleans fired the first shot when they annexed Ala Mhigo 20 years before (Ala Mhigo after all is a part of Eorzea, so it only makes sense to be included). So we didn't really become "the aggressors", Eorzea simply finally started to mount a significant counter-attack against the Garlean invasion - it took over 20 years, but hey, better late than never!