Gimai Seikatsu - Days With My Step Sister - Chapter 124: July 22nd (Thursday) — Asamura Yuuta

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 order to avoid marrying an old man from the neighboring county, she must run away. The first time she escaped, the general entered the city, and she rolled and fell in front of him, no, she kneeled in front of him (really not on purpose). The second time she escaped, the general came to visit her house, and the general caught her (he must have done it on purpose). The third time, she was bruised and battered, almost desperate, and he came galloping and took her into his arms. (He was very well-intentioned.) The general said, ” Although I do not want to hold a favor to you, it is true that you owe me a favor.” So? 

 An Ruochen is unwilling to submit to fate and meets General Long three times when she runs away from home. She expects to get help from a powerful man, but she is involved in a secret conspiracy instead. Vicious family, a missing younger sister, a mysterious spy, and power struggles. An Ruochen becomes the key to solving the mysteries and becomes the core of General Long’s heart. 

One sentence synopsis: The right person at the right time changes everything.

- Description from Novelupdates

“Do you like me?” “Yes...” “When did it start?” “A long time ago...” “Will you always like me?” “The fireworks...will always be with you...”Looking at her upturned face, the girl's timid hand was gently lifted up. The warmth in her palm has always been so familiar. “Want to go watch the fireworks together?” “Yes.”The summer fireworks blooming in the deep night are just like your touching and tender smile twelve years ago.----------------------------------PS: Youth daily life story, childhood friend single female protagonist.

In his previous life, the MC was a total degenerate—lazy, self-indulgent, and only skilled at gaming and skipping responsibilities. One day, he dies in a ironic accident.He wakes up in a magical world, reincarnated as Eryndor Sylvaren, the youngest prince of the Ethereal Elven Empire. The elves are a proud and noble race, and their royal family is bound by strict traditions of honor, discipline, and responsibility—none of which the MC has any experience with.Luckily, he receives the “Divine System of Degenerate Growth,” a system tailored to his flawed personality. He makes the same mistake of repeating his past life only to realise that this new life has so much in store for him. Mistakes have consequences for power.NOTE: This book is a slow paced action adventure story that explores a vast fantasy world. If you like weak to strong, harem growth, high stakes, dungeons and adventurers,with a compelling storyline you should give it a try.