@Yautja There's still a problem with that, the initial attack where they sent 20~30k soldiers through the gate got decimated. That was what cased the JSDF to come through the gate and bunker on Alnus in the first place. When the Japanese came through the gate and set up shop, do we really assume that the empire just let them do it without any resistance? Meaning that that initial assault shown in chapter 1 after the attack in Japan had to happen during the points where the hill still wasn't completely fortified (again going from the manga alone, the anime had some fortifications). If the hill wasn't fortified, then 100k soldiers attacking it should have been able to rush it and at least cause some Japanese casualties. If the hill was fortified, there is absolutely no reason for the army to rush all 100k soldiers up the hill when they know for sure that the Japanese are strong enough to wipe out the initial battalion sent through the gate. Good tactics would deem that you would send a small detachment of troops, maybe 5k, to gauge the strength of the enemy and look for holes in their defense. Charging 100k troops up a hill without any preparation or knowledge of what is going on is just suicide and something any strategist would avoid. So the only assumption we can make is that this empire; one that is based off of the Holy Roman empire, and has experience in raiding other worlds through gates if the story is to be believes; has no strategists and relies on brute force to win all of their battles. Yet they know enough about tactics that after they lose, the burn down all of the surrounding area and poison the wells so that the enemy troops can't advance.
Even if they pushed forward with the assault on the hill without any information the whole, "Surprise, your 20k vanguard unit is all dead", should have been enough for them to say "whoa, wait a minute. Perhaps we should take a step back and reassess the situation". Nope, instead they lost so many soldiers on their initial push that they didn't even have enough military strength left to defend their own country, so they had to con the unified armies of all of their smaller surrounding neighbors to come in and get themselves slaughtered in a similar manner so that the emperor wasn't overthrown.
It's completely nonsense for a country that relies entirely on military might to act in that way, but it makes for an entertaining opening to the story. How did they manage to survive previous gate openings without tactics or strategy? Heck the proper order would have been to order the Unified army to attack the hill first in order to gauge the enemies strength. Sure it would have delayed the assault, but he wouldn't have been risking his empires troops to the point that they couldn't defend themselves.