Baka to Test to Shoukanjuu‎ - Volume 1, The Fourth Question

Wang Cheng: Boss Chu, I think my mom will not agree two hundred percent with our affair, so I have a once-and-for-all proposal, do you want to hear it?

Chu Yifeng: Let’s hear it.

Wang Cheng: It’s very simple, why don’t we break up and not worry about anything?

Chu Yifeng: What you said is quite reasonable, but we can discuss this matter further in the bedroom.

Wang Cheng: …I’m just kidding. Look at my sincere face. How could I break up with you?

Chu Yifeng: It’s too late.

- Description from Novelupdates

For Marco, Lucas is his meek little brother. For the rest of the world, he is the menacing Ghost of Vermil.“Angels endow blessings. Demons sow curses.” A hundred years ago, angels and demons descended on earth. It is said they are still here.Annointed with potent holy power, Marco is set to join the Kingdom's Holy Army to fight off the demonic legion. But before that, he must confront the trail of blood and ash that points to Lucas's involvement with the dark side.As mysteries unfold and loyalties fracture, Marco is faced with the question: where does holiness end and damnation begin? If all lines are blurred and the fate of humanity is at stake, will he have enough resolve to strike down the brother he once swore to protect?

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.

She was stupidly in love with him. Because of him, she became rude and unreasonable, overbearing, jealous, insane. But he did not spare a glance to her. Married for two years, he didn’t favor her once, letting her become the laughing stock of the whole world. Eventually, she ended up in an affair, leading to her tragic death.