Upon Returning Home As A Military Major General, I Was Told To Attend School, Even Though It's Quite Late Now - Chapter 93: Why Are Girls’ Warm Nightwear ASo Fluffy?

A scientific genius named Thomas Harrier was transmigrated to an imperial prince body after a car accident that took his life. He found out that he is in a world similar to our own with a timeline akin to early twentieth century. The body that he is occupying was the imperial prince of a failing empire who suddenly rose to power after a successful assassination attempt of his father. Unaware that the attack is just the beginning of what is to come. He faces many challenges throughout. Will Thomas Harrier be able to live his second life in peace while running an empire and getting it back to its former glory? Or will he succumb into a conspiracy lurking within the country's borders?

Expect some:

Cold war era techs.

Super battleship with advanced electronics, propulsion, and fire control systems?
Helicopters?
Missiles?
Modern medicines and equipment?
AC-130?
A nuke?
Challenger or Abrams or Leopard?
F-4 Phantom or MIG-21?
Colored TV's.

Everything!!!

PS. Cover not mine, I just found it on google.

One of the world’s top killer who was fed up with his life tried to retire and live as a security guard at Jiang Yuan University. He only wanted to adapt to the life of an ordinary citizen and blend into the society. However, he didn’t expect that a random encounter with a beautiful girl, who was also the prettiest girl in the university, will become the key to a fierce pursuit, and at the same time, all kind of beautiful women, as well as powerful enemies came into his life.

Let’s watch over the journey of this Versatile Security Guard. Let’s see how he will obtain various beautiful flowers and eliminate his enemies and step on the head of powerful villains.

I only wanted to be a small security guard, Xu Taping said while lying in the lap of a beautiful flower.

- Description from Novelupdates

The romantic comedy light novel series centers on the gaming lives of various high school students who play video games, including: Keita Amano, a lonely young man who loves video games; Karen Tendō, the beautiful president of the video game club; Chiaki Hoshinomori, who constantly fights with Keita; and Tasuku Uehara, who puts on a facade of being satisfied with his life in the real world, but he in truth loves video games.