I Became the Devil Queen's Heart Demon - Chapter 117 - 88: Outsourcing the Case! Plum Rain!

Transmigrate to the world of One Piece, reincarnate as Saint Imu; Control the world government and sit on the throne of the void.He obtains the star map of the omniverse and contemplates the sea of dimensions.Gather all individual combat power, military power, financial resources and all material resources to open the star gate, conquer all dimensions, and break the limit of individual power!Unite or be destroyed, how to choose?

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.

Wazu was an ordinary townsman who shut himself off in the “mountain” because of an unfortunate course of events. After 2 years, he decided to come down from the mountain to meet with people. But there were various misunderstanding about the “mountain” which he shut himself off in. And then he repeatedly had encounters with people. And after that. . .

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.