Why does Kim Jong Un want nuclear weapons? Experts weigh in

SEOUL -- Days after a massive underground bomb blast, there are signs North Korea may be preparing a new missile launch.

South Korea has responded with another show of force: This time, its Navy conducted live fire drills, vowing it could bury its northern neighbor at sea.

But the North's weapons program continues to accelerate. Kim Jong Un has launched 18 missiles this year.

Despite the cartoon character stereotype of Kim Jong Un, Andrei Lankov, a professor at Kookmin University in Seoul, says Kim is "not a madman."

Lankov has followed Kim since he took over at age 27 in 2011. Kim consolidated his power by killing his uncle, and reportedly having his elder half brother assassinated at a Malaysian airport.

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While experts don't think Kim Jong Un will launch a preemptive strike on the U.S. or South Korea, the bigger question is: What will the U.S. do if North Korea fires another missile over Japan?

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"He is ruthless and homicidal exactly because he is rational," Lankov says. "It's not a job for somebody who is shy of killing people."

Lankov says Kim wants nuclear weapons so his regime can't be overthrown by the U.S. with help from the South.

"It's very clear what they want: It's to achieve the final victory, which means unifying Korea on their terms," says Daniel Pinkston, an expert on North Korea's weapons program. "Secondly, it means expelling the United States from the region -- and they continue to develop strategies every day to achieve those goals."

While none of the experts we've talked to believe North Korea will launch a preemptive strike on the United States or South Korea, the bigger question is what the U.S. will do if North Korea fires another missile over Japan -- that could come as early as this weekend.

None of the experts we've talked to thinks North Korea would make a pre-emptive strike on the U.S. or South Korea, because that would be suicidal. Instead the weapons are meant as a deterrent -- even as a war of words continues.