SEOUL, South Korea — The last time South Korea is known to have plotted to assassinate the North Korean leadership, nothing went as planned.

In the late 1960s, after North Korean commandoes tried to ransack the presidential palace in Seoul, South Korea secretly trained misfits plucked from prison or off the streets to sneak into North Korea and slit the throat of its leader, Kim Il-sung. When the mission was aborted, the men mutinied.

They killed their trainers and fought their way into Seoul before blowing themselves up, an episode the government concealed for decades.

Now, as Mr. Kim’s grandson, Kim Jong-un, accelerates his nuclear missile program, South Korea is again targeting the North’s leadership. A day after North Korea conducted its sixth — and by far most powerful — nuclear test this month, the South Korean defense minister, Song Young-moo, told lawmakers in Seoul that a special forces brigade defense officials described as a “decapitation unit” would be established by the end of the year.