Maou-sama no Machizukuri! ~Saikyou no Danjon wa Kindai Toshi~ - Volume 3 Chapter 16

As the era of dragons gradually came to an end, as arcane masters left countless riches and legends under the starry sky, one awakened youngster after another began to set foot on their journey.

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.

A data loving young man from Earth has an accident which transports him to a dark land filled with terrifying creatures and magical powers. Fortunately for him, he has a Biological Assistance Chip to ease his path. However, things in this bizarre and strange world are not always as they seem, and, though it is a tremendous asset, the chip doesn’t always work as expected. With all of the crazy creatures and magical mayhem around him, one can only wonder what the future holds for this young man…

- Description from novelupdates

Rise of the Arms Dealer in the World War tells the story of a man from the modern world, thrown back in time to a pivotal moment in history. Armed with knowledge of advanced weaponry and the sharp instincts of a seasoned weapons dealer, he wastes no time in exploiting the chaos of historical wars and political upheavals.Building an empire not just in arms but across industries, he crafts vehicles, machinery, and technology far beyond the reach of his time. With each calculated move, his influence spreads like wildfire, positioning him to control the tides of conflict.After World War I, he seizes the opportunity to buy up factories and cutting-edge technologies from the defeated nations, biding his time to prepare for the next global war. His eyes are set on World War II, where he plans to sell weapons on an unprecedented scale, shaping the course of history.A villain at heart, he is cold, calculating, and without a shred of principle. He deals to whoever bids the most, indifferent to the consequences. As his empire grows, so does the power he wields, but with it comes a relentless struggle to keep the forces he's unleashed in check.