MH is as far as I know, smaller than us. The Ymaryn has numerical and material superiority, but with logistics being shit, we don't know if it's enough to matter.
I'm pretty sure we've never actually been that far out, since the Mountain Horse are the remnants of the Thunder Horse that didn't get fucked by the Meteor.
We know that they were essentially a confederacy of city states out in the Steppes that constantly fought with each other, and we know that they were one of the strongest polities in the Lowlands Rumble for quite a while.
Considering it's been centuries since we've heard anything from them, they likely consolidated around a martial King and decided to loot us for all our stuff or something similar, since I imagine the Thunder Twins, who have a large population density but in comparison relatively underwhelming defenses, look like a juicy target for slave raids and such.
Having our subordinates "hold the line" honestly isn't enough for us, not right now with our army weak and our best and most readied troops either terribly weakened or already deployed in another civ's fights. That strategy is a good one, but it requires us to have a solid army ready and able to roll over our enemy if we just have the time. I think we've just gotten too used to having had such a thing for so long--i know thats part of why i didn't think to suggest/push for more martial this turn. We dont even have the best situation for quickly calling up and mobilizing a lot of troops either, with our wealth so unusually low. We can fix that next turn...but by then i'd be very surprised if we hadn't started taking stab hits from lost territory--if i recall correctly, the Khemetri war showed we took those even with "just" subject lands being taken. Not to mention the possibility of other civs jumping in on the matter while we're distracted, or the existing pirate war and the natives attacking amber road. This is honestly the most vulnerable to external wars we've been in a long time, i think.
This is why I'm hoping that it's actually a Great Power War, since those let us get swole super quick and would easily let us go onto the war footing without having to spend wealth raising armies.
The most likely scenario is that the Thunder Twins will be teetering on the edge of being taken over, just like Gulvalley was for the Khemetri, and we'll have a turn to get troops over there before the land is taken and we go into a stability spiral.
Now that I remember it, we were able to get troops into Txolla extremely quickly during the Khemetri War, so maybe our wartime logistics aren't as bad as we think?
I'm mostly just trying not to be pessimistic before the update even drops, and I prefer to think of good, or at least not bad, scenarios instead of being all doom and gloom.
In other good news, since Heaven's Hawks was in a position to send war missions against the Thunder Speakers back in the day, I wouldn't be surprised if they could send war missions to support them against the Mountain Horse.
They are our only march, so they have to have some good martial scores, right?