South Korea’s foreign minister reports that Washington plans to guarantee his nation’s defense against a nuclear-armed North Korea in writing. The promise reportedly will be formalized when South Korean President Lee Myung-bak visits the United States this week. It’s a bad idea. Washington should be shedding defense responsibilities, not increasing them.

More than a half century after the Korean War, the Republic of Korea (ROK) remains surprisingly dependent on America. It’s as if the United States was cowering before the Mexican military, begging its friends in Europe for help. In fact, the ROK requires no assistance to defend itself from conventional attack.

The so-called Democratic People’s Republic of Korea (DPRK) has a strong numerical military advantage over the South: about 1.1 million personnel under arms, compared to fewer than seven hundred thousand for Seoul. Pyongyang also has impressive numbers of other weapons, including more than four thousand tanks and roughly eighteen thousand artillery pieces.

However, most of the North’s equipment is decades old, a generation or two behind even that of the long-gone Soviet Union. Training is minimal and many of the DPRK’s military personnel perform construction and similar tasks. The Korean peninsula’s rugged geography favors defense. Putting thousands of antiquated tanks backed by hundreds of thousands of malnourished soldiers on the move south would create a human “turkey shoot” of epic proportions.

Anyway, the ROK’s numerical inferiority is a matter of choice, not an immutable artifact of geography. In its early years the South’s resources were sharply limited. But today, South Korea is thought to have upwards of forty times the North’s GDP. Seoul also possesses a substantial industrial base, sports high-tech expertise and enjoys a sterling international credit rating. The ROK’s population is twice that of the North. South Korea could spend more than the equivalent of North Korea’s entire economy on defense if the former wished. But it hasn’t wished to do so, preferring to rely on Washington instead.

The time for subsidizing wealthy allies has long passed. The financial crisis makes it imperative that the United States return to such nations responsibility for their own defense. Undoubtedly an American withdrawal would result in a far-reaching debate among South Koreans over how much they felt threatened by the North and how much they believed necessary to spend in response. But that is precisely the debate they should have had years ago. The prospect of a nuclear North Korea obviously is more frightening than even one with ample numbers of artillery pieces targeting the city of Seoul. But there is little reason to believe that the North has any deliverable weapons at this point.

Given present course, that time is likely, but not certain, to come. However, South Korea has time to prepare. Rather than relying on America for its protection, Seoul should invest in missile defense and enhance its air-defense capabilities. The South also should consider creating a conventional deterrent: the ability to respond to a nuclear strike by eliminating the Kim regime. That means developing potent offensive missile and air attack capabilities. (Japan, despite its quasi-pacifist constitution, should do the same.)

Such forces would help fulfill a second function: deter an aggressive China, if Beijing ever changed its policy from the oft-repeated “peaceful rise” to a more belligerent stance. The People’s Republic of China (PRC) has much to gain from stability in East Asia and has worked to assure its neighbors of its peaceful intentions. However, the future is unknowable. The best way for Beijing’s neighbors to ensure China’s rise is peaceful is to maintain armed forces sufficient to deter the PRC from considering military action.

Such a “dual use” capability would benefit the United States as well. The objective would not be a high-profile attempt at containment, but a low-profile capacity for deterrence, relieving Washington of any need to intervene. Most important, America should not reflexively extend its “nuclear umbrella” in response to the future possibility of a nuclear North Korea. Doing so would inevitably deepen American involvement in regional controversies, potentially turning every local dispute into an international crisis.

Moreover, while such a policy might seem to be a convenient and cheap means of protecting friends while discouraging proliferation, it would become problematic once another nation gained the ability to strike the United States. If North Korea eventually marries a nuclear weapon to a long-range missile, Pyongyang still won’t strike America. Kim Jong-il is evil, not stupid; he wants his virgins in this life, not the next.

However, such a weapon would give him a deterrent capability, modest though it might be, diminishing the credibility of Washington’s threat to intervene. The question would become: for what are American policy makers willing to sacrifice Los Angeles? Seoul and Tokyo? Sydney and Taipei? Jakarta and Bangkok? All of them?

Even if the risk was small, the cost would be catastrophic. And the U.S. government’s principal responsibility is to protect American lives, not to guarantee the security of foreign lands. Adopting a policy inviting a nuclear attack on the American homeland violates that duty. Offering nuclear guarantees also diminishes the threat — to North Korea and China — of America’s friends developing independent nuclear deterrents. Far better for Washington to indicate that it is not inclined to leave the DPRK with a nuclear monopoly among smaller powers in East Asia. While the United States would not encourage its allies to exercise the nuclear option, it should suggest that it would not stop them.

Pyongyang might not mind the further spread of nuclear weapons, but Beijing certainly would not relish the prospect of Japan, and even worse Taiwan, exercising the nuclear option. Even if the PRC was not certain that Washington was serious, it would have an incentive to bring greater pressure on the North. And that, of course, is the ultimate goal: halt proliferation to the DPRK.

Whether America and allied states would want to go down this path if Pyongyang proceeded unimpeded could be addressed then. But a written pledge now by Washington to defend South Korea against a nuclear North would eliminate perhaps the most powerful way of sharing the nuclear nightmare with China, and thus encouraging it to act against North Korea. There’s nothing unusual about American officials pledging to protect the South. Last month, Secretary of State Hillary Clinton stated: “I want to underscore the commitments the United States has and intends always to honor for the defense of South Korea and Japan.” But the justification of such a policy long ago disappeared.

Washington should devolve responsibility for the ROK’s defense to the ROK. Seoul can protect itself against conventional threats. South Korea could respond to nuclear weapons in the North by raising the possibility of building a countervailing nuclear capability. That’s not a good solution. But it might prove to be the best of a bunch of bad options.