Gate

Gate: Jieitai Kanochi nite, Kaku Tatakaeri (also known as Gate: Thus the JSDF Fought There!) is a military fantasy isekai anime.

Quick synopsis: In modern-day Tokyo (specifically Ginza), a massive mysterious portal called "the Gate" suddenly opens, unleashing an invading army of medieval-style knights, monsters, and fantasy creatures from another world. They rampage through the city in what's known as the Ginza Incident, killing civilians.

The Japan Self-Defense Forces (JSDF) quickly counterattack with modern weapons and technology, easily repelling the invaders and pushing through the Gate into the "Special Region"—a fantasy world filled with empires, magic, elves, dragons, demigods, and more.

The story follows Youji Itami, an off-duty otaku JSDF officer (and total anime/manga fan), who becomes the laid-back leader of a reconnaissance team sent to explore this new world, establish contact, negotiate peace, and deal with threats. While the JSDF's advanced firepower gives them a massive advantage over fantasy-level opponents, the series mixes action, diplomacy, cultural clashes, political intrigue (including other Earth nations wanting access to the Gate), and some slice-of-life/harem elements as Itami ends up with a group of fantasy girls (like an apostle demigoddess, a mage, and an elf) joining his squad.

It's basically "modern military vs. fantasy world" with Japan as the "conquering" force, heavy on JSDF propaganda vibes, gunfights against dragons and armies, and Itami's chill otaku personality driving a lot of the humor and interactions.