The Retired Hero is an Extra - Chapter 17 - : Midnight Irregularity

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.

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

A boy wakes up with omnitrix as a X-GENE