While the Pentagon is always planning for contingencies, it was particularly prescient in its choice of war games the week before North Korean dictator Kim Jong-il's death.

As one of its three possible doomsday scenarios, the US Army selected “The collapse of North Korea” at its Unified Quest exercises. In the scenario, North Korea's collapse comes about because of regime change in the isolated nation.

Kim Jong-il's death, announced Monday, raises questions about whether his son and appointed heir, Kim Jong-un, will be able to consolidate power.

Pulled from a paper published by Bruce Bennett and Jennifer Lind at Harvard University’s Belfer Center for Science and International Affairs, the scenario begins with the Kim regime "embarking on the most difficult challenge that such regimes face: succession."

“The transition from apparently stability to collapse can be swift,” the scenario says, and “could unleash a series of catastrophes on the peninsula with potentially far-reaching regional and global effects.”

Among the potential effects would be a massive outflow of the nation’s 24 million people, many of whom are severely malnourished, across the border. Equally, if not more troubling, would be the security of North Korea's arsenal. “North Korea’s weapons of mass destruction could find their way out of the country and onto a global market,” the authors say.

While war games are useful preparation for the worst eventualities, the Pentagon has long been planning for Kim Jong-il’s death.

The 2010 Quadrennial Defense Review – the Defense Department’s top strategy document – warns that “stability or collapse of a WMD-armed state is among our most troubling concerns.”

But “to this point, we have not seen any change in North Korean behavior of a nature that would alarm us,” the nation's top military officer, Chairman of the Joint Chiefs of Staff Martin Dempsey, told reporters traveling with him on a trip to the Middle East.

He learned of Kim Jong-il’s death “in the middle of the night,” he said. While he quickly consulted with “the chain of command,” the decision was made not to put US troops on heightened alert.

“No changes in troop dispositions, no changes in readiness levels,” he said. “We’re simply remaining vigilant.”

• This article has been excerpted and updated from:

Doomsday war games: Pentagon's 3 nightmare scenarios