Tate no Yuusha no Nariagari - Chapter 11 - Accomplishments of a Slave

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.

Li Xu occasionally got a sign-in system, and his life has been different from now on. Enjoy life, look at the scenery, sign in, spring flowers bloom, and face the sea.

- Description from novelbuddy

A man is reborn into the world of Avatar: The Last Air Bender as a waterbender from Katara's tribe. Best of all, he was the same age as her, and he had a system with him to help.He knows the future and a system that lets him continuously get stronger by completing quests or long-term missions, as well as giving him the ability to copy abilities from others for himself. He will become invincible in the world of Avatar, even more so than the Avatar himself.

As the second daughter of Ningyuan Army General, Qin Wanru, who had been set up by her so-called mother and elder sister, lost her reputation and suffered a lot on her wedding in her previous life.

Now she was reborn and returned to the time before she married to Qi Tianyu, the first son of the magistrate of Jiangzhou.

The rebirth gave her a chance to revenge on her own wedding and uncover the plot of her mother and elder sister. Would she be able to change her fate?

- Description from Novelupdates