Interesting that the screenshot with Gaius seems to depict him in Terncliff's countryside. The sky has the same coloration as the Terncliff countryside flashback during the Emerald Weapon fight, and there is also what appears to be the same kind of tree too.
The storyline has been building up to a final showdown with Valens at Werylt, so I'm guessing that this screenshot is from when the Alliance is marching from Terncliff towards Werylt on foot? Gaius seems to be pissed at something, so I'm guessing that Valens send forces to harass them along the road? He doesn't seem to be looking too high up so it can't be an airship bombing run.
Also, the fact that the weapon storyline isn't mandatory seems to imply that that the Alliance won't be invading Garlemald.
In order for the Alliance to push into Garlemald, they need to reach Ilsabard. To reach Ilsabard, they can either 1. cross the Ghimlyt Dark on foot, or 2. sail by ship from Eorzean ports to a landing zone in Ilsabard. There is absolutely no reason to pick option 1 over option 2; water transportation is simply more efficient to move people and supplies. Right now, the Alliance's foothold in Ilsabard is Terncliff, which is a sea port. So if the Alliance is going to invade Garlemald, it's going to be through Terncliff.
Except Terncliff is a location exclusive to the Weapon storyline, which is optional. So unless Square decides that they're going to do some weird stuff where the MSQ has the Alliance invading through Terncliff, but it's possible that the player has never even done the weapon storyline and didn't even conquer it in the first place, then... I'm guessing that the Alliance just isn't going to invade at all. Maybe they will send a few airships carrying technicians or something to help the Scions out, but no military campaign.
Aw. Disappointing.