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.

This series contains examples of:

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.
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 aptahly 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 convince the author to remove or tone down those views since Japanese Right Wing politics are controversial and very sensitive in Japan.
The Empire
Names to Run Away From Really Fast: Rory the Reaper
Otaku: Youji is an Otaku and his ex-wife Risa is a doujin artist.
Our Elves Are Better: Elven archer Tuka Luna Marceau.
Shown Their Work: Takumi Yanai is a former JSDF reservist so he knows his weapons very well.
The Stoic: Sorceress Lelei la Lalena.