A gate to a medieval fantasy land opens up in Japan, and this can only mean one thing: sword-wielding troops get mowed down by the glorious Japanese military in their attack copters with Wagner playing. And I mean that in-universe; the soldiers are literally playing a CD of Wagner in their copters.
Why, you ask? Because Japan needs more technology and minerals and such because nationalism says so. Oh, and some humanitarian reasons, but those aren't as important.
The plot kicks off when out of nowhere a gate appears and from it come a bunch of orcs and medieval troops that indiscriminately kill civilians, wreck the place and interrupt the otaku event the protagonist was planning to visit. As so it happens, the protagonist is not only an otaku but also a member of the SDF (Japanese military), and once the initial attack has been dealt with, he joins the force sent through the gate.
This fantasy land is declared a part of Japan, named the Special Region, because why not? (Nice name, by the way; that's what I'd call a fantasy land too if I ever occupied one.) The Japanese constitution forbids (at the moment, anyway) deploying the SDF abroad, but I guess that's one way around it: just annex territory arbitrarily and suddenly it's all Japanese soil. Some fighting ensues, but no worries: their enemies are so outgunned they might as well be throwing rocks at them. Aside from the technology gap, these feudal lords also come from the Zapp Brannigan school of tactics with plans like:
1. Suicidal charge,
2. Suicidal charge, and
3. Suicidal charge under the cover of night (worth a shot, I guess?)
Actually, that was kind of the idea behind the original invasion into Japan: go to an unknown land without any intel or recon and slaughter everything for fun and profit. ...Yeah.
Soon enough our merry band can venture further into this land while talking about their waifus and singing magical girl theme songs, eager to meet the local catgirls. In the process they recruit mages, elves and goth lolis. The writers thought the SDF wasn't overpowered enough, so the goth loli happens to be an invincible demigod priestess who effortlessly crushes everything in her path. The tone of the show is what you might expect, with slaughter and fanservice taking turns. What is a hot bath scene without people's arms flying off afterwards?
There is also a cultural clash going on, which is used for dialogue like:
"I had no idea our world had [insert everyday thing here]."
"Princess, this is another world!"
Repeat it a few times and you have comedy gold.
Some of the cultural misunderstandings actually make sense, such as the different interests of a feudal society and a modern country, the treatment of prisoners or the idea of throwing a half-naked woman at the protagonist to earn his favor. Actually, the last one just might have worked if he hadn't been distracted by all the animal girl maids already. And who can blame him for being distracted? Barely dressed girls keep throwing themselves at him out of coincidence or because he's just that awesome.
If it isn't obvious by now, the operation beyond the Gate is a one-country effort. Sharing resources with others? Don't be ridiculous. The US, Russia and China (the rest of the world doesn't exist) don't deserve their slice of the cake. Especially America. "A sky with no civilian or US planes in it is a dream come true" for Japanese fighters.
Of course, this is all run by the SDF. Civilian personnel? Media presence? Dream on. All we need is the SDF. Who else would defend Japanese interests? The Japanese government is corrupt and easily swayed. The parliament is a bunch of idiots who want to mess with the SDF's business for no reason. How dare they ask questions about civilian casualties under a media blackout? Eventually there is an entire town for the locals, with shops and everything. Would now be a time to bring in civilians? No, that could get in the way of the runaway militarism we have going on. To maintain order, some of the locals are actually given armbands with "Military Police" on them.
The visuals include machine guns, missiles, tanks, artillery, copters and fighters, which is nice. Sadly most of the soundtrack is not Wagner, which is a shame because that would be poetic justice.
By the way, it is never explained how the Gate was built. Did the Empire build it (which I kind of doubt) or just find it? Are there more of them? If you are after fantasy tech, shouldn't this be near the top of your list? Think about it: you could discover a Stargate network and explore even more worlds for even more glorious nationalism! But nope, never mentioned.
Now let's all take a moment and salute the Japanese flag flying in the distance.