One Piece : Start As Celestial Dragon And Take Down Hancock - Chapter 102: Warm welcome (R-18)

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.

I’m a policeman, we took over the crime scene, and all the idlers and other people flashed!

Are you a detective? Do you have a formal license? Is it part of the police system? May I list you as a suspect?

Detective? Sorry didn’t hear it.

You want to reason? I’m sorry I’m not interested, but I’m standing there watching us handle the case!

Hey! Don’t move around at the crime scene, it’s you, that kid over there!

——New scum, listen to the teaching

- Description from novelbuddy

In a world where beast masters rule by taming magical creatures and competing in thrilling tournaments, becoming the most celebrated sport, Eratz seemed destined for greatness. Surrounded by friends and family, his path was set until a shocking betrayal shattered his dreams.Fueled by the pain of his betrayal, Eratz looks up at the night sky, finding a new purpose. He vows to rise from the ashes and conquer the beast master world, determined to become the greatest and reclaim his lost honor.

Traversing into a Fantasy World, Watson became the youngest son of a run-down baron, ranked eight in the family.