I'm the Mysterious Leader of the Salvation Organization - Chapter 497:

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.

In a world where dungeons loom like dark secrets and monsters stalk the land, Rem’s awakening shattered the status quo. A surge of raw power flooded his veins as the Monster Monarch System bound itself to him, unlocking a terrifying potential.Each heartbeat echoed with the promise of countless souls waiting to be absorbed—skills, abilities, and monstrous strength ready to be claimed.Fuelled by an insatiable thirst for power, Rem set out on a deadly quest, driven by one singular goal: to rise above the chaos and stand at the pinnacle of the world. With nothing but the deadly abilities granted by the System, and a burning will to crush any who dared stand in his way, he carved his path through savage lands and ruthless mages.The hunt for strength never ends. The battle for supremacy has only just begun. Rem’s eyes are set on the throne of the Monarchs, and the Monster Monarch System will ensure nothing stands between him and his destiny.The grind was eternal...

This is a time of great powers: The vast Persia, Sparta that is founded by force, Athens with its trade and business, Thebes the rising star, The ambitious Macedonia, The twin heroes of the Western Mediterranean, The rising Rome and so on.

This is an era of brilliant stars: Agesilaus II – the last king of Sparta, Epaminondas – the Rising General of Thebes, Philip II – the founder of Macedonian hegemon, Dionysius – the tyrant of Syracuse, Camillus – the Roman dictator and savior…

This is an era of contention among a hundred schools of thought: Plato – the great philosopher, Antisthenes – founder of cynicism, Pythagoras – school of number, Democritus – the encyclopedia scholar, Hippocrates – the father of western medicine, Aristophanes – the great dramatist…

What will the hero bring to the world when he comes to this age as a humble mercenary through rebirth?

- Description from Novelupdates

In the distant future on Earth, there was a girl with God-like powers. That girl is the last survivor of the former human race that flourished in ancient times.

The Ancients.

And on that very same Earth, a new breed of humans with the ability to wield Magic emerged.

That girl began to observe the new human race silently.

Sometimes called a God, other times called Satan, that girl continued to observe the new human race.

This is the story of the happenings between the lonely girl and the hardworking new human race.