The Ascians are the true threat. Varis has been shown to have very little desire to launch an attack upon Ala Mhigo. That, to me, suggests that the attack in 4.5 will be a result of Elidibus/Solus going behind his back or simply slaying/imprisoning him to seize total control of Garlemald. Another possibility is that the Ascians unleash Black Rose upon everyone fighting on the battlefield, forcing the survivors to retreat and lick their wounds. The Ascians have no real loyalty to Garlemald, so they'd be unconcerned with sacrificing many of them to serve as the 'mass death' that would bring forth a Calamity.

I'm of the belief that we won't be exploring Garlemald properly until 6.0 - and by that point enough rapport with the Garleans will have been built up to justify the Warrior of Light and some key allies venturing there to help put an end to the Ascian scheming once and for all. In other words, 5.0 will likely be heavily focused on diplomacy and negotiation in a desperate effort to prevent a full scale war from breaking out. It's worth noting that the patch preview site shows Raubahn sitting on a chair with the symbol of Garlemald etched into it, suggesting that he and the other leaders pictured with him are talking things out. Whether that happens before or after the dungeon remains to be seen, though.