Hero? I've Long Stopped Being One - Volume 1 Chapter 23

When a glorious old civilization was toppled overnight, a strange purple moon rose and thus began the Purple Moon Era.

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.

After dying, Chen Heng finds that he has transmigrated to an alternate reality: everything is the same, except the world is now a Wuxia world heavily influenced by martial arts! Not only this, he discovers that he has a Fantasy Simulator, from which he obtains rewards that make him incredibly powerful in his new reality as well. Martial arts, immortal cultivation, and overpowered abilities–all of this begins from a single simulation!

After his soul transmigrated into the world of Teyvat, Luo Qing was selected to become the new Trainee Fatui Harbinger, and he was about to be sent to Liyue on a diplomatic mission (to cause chaos). To his surprise, he awakened a “Frustration Value Collection System” — as long as he frustrated others, he could collect frustration points, which he could use to participate in a lottery.As he stared at the list of items in the lottery interface, Luo Qing fell deep in thought:“Spirit Speech: Time Zero,” “Multi-Shadow Clone Technique,” “Thunder Breathing Style,” “Golden Experience Requiem,” “Railgun,” “Gomu Gomu no Mi...”Why on earth were these things appearing in the world of Teyvat?!Given the notorious reputation of a Harbinger, Luo Qing knew that if he didn’t grow strong, he was certainly doomed to a miserable end. Thus, his style became... increasingly abstract...---Years later, to prevent the infamous Fatui Harbinger, Luo Qing, from causing havoc everywhere, the people and political leaders of the Seven Nations united to issue a formal protest to Snezhnaya.In response, the Fatui in Snezhnaya issued an official statement:“This Harbinger is only a trainee. He has been dismissed.”