Rejected After One Night Stand, Desired by the Billionaire - Chapter 3 - : He Has a Mistress

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.

The moment the little tyrant of the nobles, Roel Ascart, saw his stepsister, he recalled his memories. He realized that he was in the world of a gal game he played in his previous life. To make things worse, he was the greatest villain in the common route of the game!

“I’ll be killed by the main character and the four capture targets ten years from now. Is there still any hope for me?”

Just thinking about the fearsome glints of those sharp swords those beautiful capture targets held in their hands, Roel couldn’t help but tremble in fear.

Till a voice finally sounded in his head.

【Welcome to the House Resurgence System】

Volume 1 (Set between volumes 13 and 14)

A series of short stories set before the events of volume fourteen including how Team GROUP was formed.

Volume 2 (Set between volumes 16 and 17)

A series of short stories including two separate meetings between Touma’s and Mikoto’s parents, two tales involving two of the Level 5s of Academy City, and other funny stories involving some of the characters.