@Mathiason I get the intent, but that is some seriously poor decision making on the Eastern Holy Nation's part. Throughout history rulers have tried to distract from domestic problems by creating a foreign enemy and that's usually ended badly. The fewer mouths to feed argument doesn't entirely work here since you require strong and healthy male labor to tend to the farms. If you conscript all the healthy soldiers and leave the old, young, and sick that's setting yourself up for failure. You suddenly have drastically less labor tending to crops. Plus feeding an army is a massive endeavor. Maintaining one is even more expensive. Back then farmers were taxed in grain. So suddenly, decreased labor force -> less crops + high taxation --> extremely unhappy conscript soldiers who know their children, wives, and parents are starving to death at home while they fight a pointless war on foreign soil.
War strains economic and political stability and reveals weaknesses in a nation. Sometimes that's enough to bring an empire to its knees. One extremely important aspect to note is that certain parts of the nation will be wealthier while others struggling. In good times, if you drum up enough war hawk patriotism, you might get some grumbling but you're strong enough to weather the criticism. The same holds true now. When war isn't fought on your soil or you don't experience significant decrease in quality of life, no one really cares. BUT when you're already under intense internal pressures, regional tensions will emerge. Say you're in a wealthy area and the empire calls on you to send more men and finances and live under ration because you're capable, you'll think hold on, why should I do this for the poorer areas of the empire (which are also often culturally, religiously, and ethnically unique). We're doing well on our own (maybe they're the bread basket region or industrial area), either tax us evenly, avoid war altogether, or let's breakaway.
This is exactly what's happening in modern Italy split between the industrial Lombardia and rural Sicilia. No one can agree on changing the tax policy. Also the Catalonian independence vote, which was successful btw. This is also why Brexit happened since wealthier nations didn't want to pay for those under austerity. And that's without the pressures of war. Meanwhile, the poor regions at wits end will straight up revolt because why not, they're going to die anyway they might as well take their chances at home. There's also a heck ton of resentment over wealthier regions sending the poor folk to war while being unwilling to distribute wealth. Then tada, you get peasant revolts + external war, major contributing factors to the fall of the Western Roman Empire, Achaemenid Empire, Tang Dynasty, and into infinity. Like that was literally how Imperial Russia ended. The Bolshevik takeover started from the February Revolution with women protesting in bread lines after the imperial dynasty decided it was more important to conscript soldiers to fight a European war. Never underestimate women in bread lines. I'm looking at you Women's March on Versailles. I repeat, never underestimate women in bread lines.