I have a feeling the Garlean Empire knew about Bahamut and had a method ready to contain him.

According to Cid, the Meteor project is a complex scheme employing arcane magicks and lost Allagan technology to summon the lesser moon Dalamud-a giant machine, if his claims are to be believed-down to Eorzea in an attempt to annihilate all of the beast tribes in one fell swoop. Seemingly in conjunction with the project, Legati Gaius van Baelsar and Nael van Darnus are currently searching for relics known as "tomestones" said to contain records of the past, despite the fact that Ironworks studies have shown these items to be worthless...
Wasn't there a tomestone or something in the Limsa quest that was used to contain Leviathan?