Reincarnated as Itsuka Shido: Collecting Spirits as Lovers - Chapter 30: Cornered Shido

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.

Henry is a university graduate who happens to be working as a hotel receptionist, though with big dreams of being more than that. All these change for good one day when he finds a way of cracking a very sophisticated technological system, which ultimately empowers him to formulate and develop leading-edge inventions from his bedroom.With a determined mind not only to conquer the market but also to move out of his gray life, Henry starts into his journey of fame by introducing a number of radical technologies that capture the interest of the whole world and hold it.

The mighty heavens exist, ruling over all things. Those favored by its will are granted power and fortune, while those who dared to oppose it perished, leaving none behind. This is the tale of a man who once defied the heavens, underwent multiple reincarnations, and returned once more to the cultivation world. But here’s the twist. This young man remembers nothing of his past lives and is simply a teenager from Earth. How will he who has no knowledge of cultivation and experience in fighting, survive this grim world while being an inner disciple of a lower realm force—or so Xiao chen, the teenager thought till he heard a systematic sound in his mind. [Ding! Host has awakened the heaven’s destined villain system!]

Roy died during a war against the Evil God Cult, but somehow, he returned back in time to 30 years in the past where he was just a boy who awaken his summoning magic.