Gate

In August 20XX, a portal to a parallel world known as the "Gate", suddenly appeared in Ginza, Tokyo where monsters and human warriors wearing ancient armor poured out of the portal, killing everyone in their way which would later be known as the "The Ginza Incident". Immediately after the attack, the Japanese Ground-Self Defense Forces launches a counter attack, repelling and pushing the invaders back to the Gate and reclaiming Ginza.

Hoping to establish diplomatic relationship with the people beyond the Gate and capture the ones responsible for the massacre of Ginza, the JSDF, under orders by the Prime Minister, takes over and enters the Gate where they dispatch the Third Reconnaissance Team to scout the "Special Region" led by 33 year old First Lieutenant Youji Itami. Itami was there when the Ginza incdient happen and luckily for him, exploring the Special Region is a dream come true since the Lieutenant is an Otaku especially after he meets some locals who are exactly like his fantasy type girls. While the 3rd Recon Team explores the Special Region, back on Earth, the major powers takes a special interest with the Gate...

Gate - Thus the JSDF Fought There (Geito - Jieitai Karenochinite Kakutatakaeri), is a Japanese fantasy novel series by Takumi Yanai. Originally serialized online on a novel website called Arcadia, it was published in book form by Alphapolis in 2010. There are 5 novels and a manga serialization started in July, 2011. An anime adaptation began airing in July 2015. The series went on hiatus after the first twelve episodes, which resumed in January 2016.

Tropes used in Gate include:

Ancient Grome: The Empire is very much based on Rome, with Latin-style names and even a Senate. Its allies and vassals however have more Medieval European Fantasy elements.
Armor Is Useless: Medieval/Roman style armors are useless against bullets.
Badass Army: The JSDF.
Badass Bystander: Youji was shopping during the Ginza incident, where he immediately took command of the police and help the evacuation of civilians to the safety of a castle and hold off the invaders until the JSDF arrive.
Cool vs. Awesome: Modern Armies versus Fantasy Creatures!
Curb Stomp Battle: After the JSDF are finally called in to stop the invaders, they proceeds to beat the crap out of the invaders with guns, tanks and helicopters. When the invaders try to attack again after the JSDF comes to their world and takes over the Gate, the JSDF defenders merciless shoots them down.
Eagleland: The Americans are helping Japan maintain control over the Gate, with the JSDF even using equipment given to them by the US military. Though US President Dirrel isn't above engaging in backroom conspiracies to make sure that America gets the bigger piece of the pie in potentially exploiting the "Special Region."
Easy Logistics: Averted, it took a week for the JSDF to reclaim Ginza since there were logistic problems to gather the troops and equipment needed while the bureaucracy were having a debate whether they should send the JSDF or not.
Elegant Gothic Lolita: The aptly named Rory Mercury.
Executive Meddling: A reasonable example. The online version of the novels had very right-wing views by the author. When Alphapolis published the printed novels, they convinced the author to remove or downplay those views, since right-wing politics tend to be a sensitive if at times controversial issue in Japan.
The Empire
Fantasy Counterpart Culture: The "Special Region" is a melange of Medieval and Classical Western influences. With The Empire in particular being a fantasy Ancient Grome, complete with its own Senate.
Godzilla Threshold: Apparently, the deployment of the JSDF into Ginza and eventually, the "Special Region" is deemed this by the Japanese government.
Make the Bear Angry Again: The Gate's appearance and the potential wealth of the "Special Region" are deemed a threat to Russia's global standing. And it's implied that the Russians are trying to find ways to destroy it.
The Masquerade: Averted. Since the Gate's appearance, it's become relatively common knowledge across the world. And various world powers are particularly interested in it.
Names to Run Away From Really Fast: Rory the Reaper.
Next Sunday A.D.: The series is set in otherwise contemporary/near-future times.
Otaku: Youji is an Otaku and his ex-wife Risa is a doujin artist.
Our Elves Are Better: Elven archer Tuka Luna Marceau. Also Yao the dark elf.
Quality vs. Quantity: The Imperial Empire may vastly outnumber the JSDF 30 to 1, but they are vastly outclassed and outgunned.
Reality Ensues: A modern military fights forces with pre-industrial technology. The results are predictable.
- While the JSDF and its "Special Region" allies are doing their jobs, major powers like the Chinese and Russians have set their sights on the Gate. And given the untapped potential of the "Special Region," neither is the US helping the Japanese keep control over the Gate purely out of altruism.
Shown Their Work: Takumi Yanai is a former JSDF reservist so he knows his weapons very well.
The Stoic: Sorceress Lelei la Lalena.
Teeth-Clenched Teamwork: Despite the US and Japan cooperating with each other regarding the "Special Region," things aren't so friendly as they appear.
Tempting Fate: The armies of the Unified Kingdoms assumed they would easily achieve victory against the otherworldly army occupying Alnus Hill just because they have the advantage in numbers, thinking such a "puny army" stood little to chance against their military might. They get instantly obliterated by the JSDF's heavy artillery, nearly wiping them all out. The armies never even gotten halfway to where the JSDF were stationed.
Tokyo Is the Center of the Universe: Justified as the titular Gate to the "Special Region" is located in Ginza. At the same time however, this drags Japan into the midst of an increasingly tense geopolitical landscape as major powers from the Americans to Chinese want a piece of the action.