(Cherie A. Thurlby/U.S. Air Force/Getty Images)

Two B-2 "Spirit" aircraft, nuclear-capable stealth bombers that are as wide as a 17-story building is tall, took off early on Thursday from an air force base just outside of Knob Noster, Mo. They flew across the Pacific Ocean, past the Korean peninsula, to a small island in the Yellow Sea, where they dropped some inert munitions before flying all the way back to Missouri.

Such exercises are rare, or at least rarely publicized: after it was over, the U.S. military announced the practice bombing run, the first time it has ever acknowledged a B-2 mission over the Korean peninsula, according to a New York Times story.

Why conduct such an elaborate exercise? A big part of the answer is, of course, as a deterrent to North Korea's recent provocations, which have included severing emergency communication lines with the South, announcing a state of readiness for war and threatening "pre-emptive" nuclear strikes on the U.S.

But there may be something more going on here. Pyongyang's latest threats are not new; although U.S. shows-of-force are part of the routine, this was an unusually dramatic way to demonstrate American deterrent capability. It's possible that the bombing test run was also meant as a message to South Korea. That would be a deterrence of a very different sort: not from war, but from the possibility that this long-reliant American ally might seek to develop its own nuclear weapons program.

South Korea has long been under the American nuclear "umbrella," meaning that the U.S. extends its nuclear deterrent to South Korean soil. But, over the last year, a small group of right-leaning South Korean politicians and opinion-makers have been arguing that their country should develop its own "indigenous" nuclear weapons. And South Koreans appear to be increasingly persuaded: a recent poll estimated that two thirds of the country supported the plan.

According to a New York Times story on the rising South Korean calls for a nuclear program of its own, the public support is less about fear of the North (although that's certainly part of it) than it is about doubts that Uncle Sam will always be there to help out:

One of the biggest is the creeping resurgence of old fears about the reliability of this nation’s longtime protector, the United States. Experts say the talk of South Korea’s acquiring nuclear weapons is an oblique way to voice the concerns of a small but growing number of South Koreans that the United States, either because of budget cuts or a lack of will, may one day no longer act as the South’s ultimate insurance policy.

A big, showy B-2 test run over the Korean peninsula probably isn't going to change North Korea's behavior very much – it's hard to imagine that Pyongyang has any doubts about what would happen if it started a second Korean war. But it might help persuade South Korean citizens and politicians that the U.S. really is there for them.

The U.S. persuaded South Korea not to develop its own nuclear weapons once before, in the 1970s, when Seoul's then-military government wanted nukes to deter the North. The U.S., then as now, wanted to reduce any nuclear weapons proliferation. Every additional nuclear state lowers the threshold and taboo for the next one, as well as raising the risks that some misstep, miscalculation or miscommunication might lead to nuclear war. And, even if none of those things happen, it's difficult to predict how China might react to having a pro-American nuclear power so close to its border.

A nuclear-armed South Korea would actually risk causing the thing that Seoul so fears, lessening the American support. The U.S. has invested so much time and energy into deterring any new nuclear development, particularly in the Middle East, that it might feel compelled to distance itself from the South if that country violates the Non-Proliferation Treaty.

Duyeon Kim, a Korea expert at the Center for Arms Control and Non-Proliferation, warned darkly that a South Korean nuclear program "would be a clear violation of international law that would sever political and economic relations with its closest ally, the U.S., and neighboring countries." With the drop in international trade, she predicted, "No more smartphones, no more fashion, no more musical sensations like Psy."

If it's a choice between an isolated and nuclear-armed South Korea versus a U.S.-supported but U.S.-reliant South Korea, then even the country's more nationalist politicians would probably choose the latter. So would Washington. And what better way to signal it than with a nice air show?