On Our Wedding Night, I Discovered the Crown Prince's Hidden Illness - Chapter 69 - : Holding Her in My Arms

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.

[Warning: This story contains extreme violence, philosophical depth, psychological breakdowns, self-improvement, self-realization, and themes of maturity.]“Even Gods need character development.”That was the belief Pratham lived by—struggling, always pushing forward. Until the day he died, starving in a world too cruel to care.But death was only the beginning.He transmigrates and awakens in a multiverse as Leon. In this multiverse, the greatest threat to humanity isn’t demons or beasts, but Entities. In response, civilizations across each universe forged a single hope: the Entity Hunters Organization, training warriors to stand against the impossible.He had no magic. Just his will, his fists—and a little sister he swore to protect at all costs.Enrolling in the Entity Hunters High School, Leon finds comrades, purpose, and war. He fights beside them across different planets and universes, grows with them, laughs with them—then loses them all.Leon rises not as a hero, but something else.

10 years ago, after “the Gate” that connected the real world with the monster world opened, some of the ordinary, everyday people received the power to hunt monsters within the Gate. They are known as “Hunters”. However, not all Hunters are powerful. My name is Sung Jin-Woo, an E-rank Hunter. I’m someone who has to risk his life in the lowliest of dungeons, the “World’s Weakest”. Having no skills whatsoever to display, I barely earned the required money by fighting in low-leveled dungeons… at least until I found a hidden dungeon with the hardest difficulty within the D-rank dungeons! In the end, as I was accepting death, I suddenly received a strange power, a quest log that only I could see, a secret to leveling up that only I know about! If I trained in accordance with my quests and hunted monsters, my level would rise. Changing from the weakest Hunter to the strongest S-rank Hunter!