Ragnarok: Alpha and Omega - Chapter 6 - 5: An Aura Unmatched

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

Roan crossed over and became an FBI agent.

Several years later, when interviewing the world-renowned detective Roan Greenwood, the reporter asked:

“Excuse me, what made you persevere on the road of solving the case?”

Roan tilted his head and glanced at the sports car beside him, the villa behind him, and the beauties in the villa, and smiled slightly:

“Of course it’s the bonus after solving the case.”

“???”

- Description from novelbuddy

Seol Woon-hwi, the Blood Heaven’s Righteous Blade, rose to power, becoming second to none under the heavens, reigning just beneath the sky itself.Yet, at the height of absolute power, he finds himself returned to his past—back to his days as the scorned illegitimate child of the Everlasting Snow Palace.