Kuro no Maou - Chapter 475

A college student was returning home after studying abroad for several years when the end of the world happened. A dragon appeared out of nowhere and attacked the plane he was in, thanks to an incredible amount of luck and his quick-thinking, he managed to survive the plane crash only to get stuck on a tropical island. A weird system took over the world and brought monsters to Earth. While nations collapsed and the world as a whole was being destroyed by ridiculous monsters, Leon was stuck on a godforsaken island with no chances of being rescued. What will happen to him? How will the world change while he is away? Will he even manage to return? The story, all names, characters, and incidents portrayed in this production are fictitious. No identification with actual persons (living or deceased), places, buildings, and products is intended or should be inferred.

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.

On the desolate barren planet of Rennes, an Asian teenager named Lin Yuan receives an admission notice for the Empire’s best military academy San Romia. So begins his life changing military school career.

Lin Yuan: Wow! This school’s really great! Everyone even has a separate bedroom! But…why does my next door roommate always knock on my door? And all day long he keeps bothering me, God! So annoying!

Lin Yuan: I’m a Beta, my biggest wish is to meet a nice Beta girl, get married, have children and live in peace! Wait a minute… Why does my body suddenly feel strange? Why does this medicine look like drugs to prevent Omega from entering estrus?

Trembling in his roommate’s arms Lin Yuan blushes and tries to push off his chest: “Wait…wait a minute…I’ve always thought of you as a friend. Don’t do this…..ah….”

At that moment, Lin yuan seems to feel that the universe has malicious plans for him. The world is collapsing, who will tell him why the script is so wrong?

- Description from Novelupdates