Spell Weaver - Chapter 94. Small Steps Make the Big Changes

He was the continent’s strongest and most revered alchemist, Ye Zifeng. He was under one person and above millions. However, due to an unpredictable event, he had died. After his rebirth, he had become the supertrash of the Ye family and suffered disgrace. To make matters worse, he had an argument with his fiancé that led to the break off of his engagement. In this second life, he will work to change his own destiny. He vows to return to the peak of the world, trample on those who laughed at him and rewrite his legacy!

- Description from Novelupdates

“Rishe Ilmgard Wertsner! I’m breaking off my engagement with you!” “Okay, I understand.” “Huh.” The Duke’s daughter, Rishe left the evening party without delay. In fact, for Rishe, this engagement has been terminated for the seventh time. That’s where her new life starts, after having enjoyed the way of life of a pharmacist and a dressed up knight, she lost her life at the age of 20 and has gone back to the scene where her engagement was broken off. Normally, this could have broken Rishe’s heart, but her mentality became strong. Her past six lives have been busy and fulfilling, she wants to live longer this time around. If possible, she wants to live a life of leisure and squander money… To commence her 7th life, Rishe has set up such a diagonal goal. However, in this life, she will be married to the Crown Prince of a militant nation, who killed her in her past life. …It’s probably better. It doesn’t matter if it’s a former rival country. In her seventh lifetime, she’ll enjoy loafing off at a castle!! Against all her determination, she inadvertently demonstrated the skills she had cultivated in her past lives and garnered attention as a brilliant Crown Princess.

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.