Save a Failed Idol's Life - Chapter 110:

Lorelai is an unwanted princess; a puppet in the hands of the royal family.Born weak and raised loyal only to her kingdom, she has no real power and is constantly abused by both her stepmother and her son.Her value lies in one thing only––marriage. Thus, she is to marry Duke Kalder, an old but powerful and influential man whose previous wives have all gone missing under mysterious circumstances.That is until she meets him.Tall, dark, strong, mysterious, irresistible, seductive.The King of Lycans. The King of Beasts. The Gypsy King.One who was once a slave now controls the biggest and strongest, most feared kingdom of the continent. And for some reason, he has his eyes set only on Lorelai.Important note: this is a historical fantasy novel and in its context, the word “Gypsy” is only used as a name of a nomad tribe and is not used as a slur.

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.

Eren Idril died as an old, decrepit, and piss-poor adventurer by the maws of a D-Rank monster. He had a lot of unfulfilled dreams and desires that couldn't see the light of the day after the light in his own eyes died down.

But fortunately for him and unfortunately for the world, that wasn't his end. Eren was somehow dived back in time when he was a mere teen, attending Lionheart Academy for adventurers.

When life gives him a second chance, Eren decides to exploit it to its fullest. Experience his vile deeds as he partakes in them without feeling the slightest bit of hesitation.

Wealthy, powerful and handsome; Gu Jingze is the cream of the crop in the entire country. Every guy wants to be him and every girl wants to be with him. His life is perfect… except that he has one tiny secret that keeps him from getting close to any woman—apparent impotency.