Maou na Ore to Fushihime no Yubiwa - Volume 3 Chapter 5 — Iriss circumstances

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.

After suddenly dying while trying to meet a deadline, Gu Huai transmigrated and became a non-human race.

This race is cruel and cold, making them feared and shunned by everyone in the interstellar world.

As the new king of the race and facing the group of parents who see him as a baby, Gu Huai said he was under a lot of pressure.

- Description from Novelupdates

If you had a chance to live again, what would you do? Earn money, hold onto a thick leg, just find someone to live a happy life with? Ha ha… Stepping on the beat of the times, riding the wind and breaking the waves, strategizing, using your knowledge to change yourself, and everything around you, come back! The reality flows to the plot, the golden fingers are thick, slow, and cool.

- Description from Novelupdates

Cal was the pride of his guild, the one that other Initiates looked up to. Then he died. So did his guild.His understanding of the world shattered before his death. His guild was nothing compared to others, which meant he was the best of the worst.When Cal was given a second chance, he wouldn’t repeat his mistakes. Who cared about being a [Mage] when it would just mean death? He would survive, even if it meant being something he sneered at in his previous life.A [Farmer].Of course, when did anything go according to plan? Never.