Call it the Korean conundrum, a question to baffle students of international relations. Why is the Republic of Korea—the ROK, or South Korea—so militarily weak?

Ever since the ROK was established in 1948, the so-called Democratic People’s Republic of Korea has posed a threat. DPRK dictator Kim Il-sung launched an invasion in 1950, which was rebuffed only after much bloodshed and with the aid of U.S. troops who remained in the South after an armistice was signed in 1953. They are still there.

In the early years the South was vulnerable. But the balance of power gradually shifted. During the 1960s the South liberalized its economy, triggering sustained growth and propelling it to become the world’s 13th or 14th largest.

As South Korea was taking off, the North was stagnating. By the late 1990s the DPRK was devastated by famine. A regime that celebrated Juche, or self-reliance, ended up dependent on handouts from Beijing.

Today there is no comparison between the two Koreas. The South is an important international player; the DPRK is a national wreck. South Korea has upwards of 40 times the North’s GDP. The ROK also has a vast technological lead, full access to global credit markets, and the political clout that comes from extensive trade and investment. The South’s population is twice as great as that of North Korea.

In short, the conditions that left the South open to North Korean aggression no longer exist. Yet South Korea remains dependent on America. And U.S. policymakers assume that Washington must defend the ROK, apparently forever.

Only in military affairs is the South’s superiority in doubt—and the DPRK’s advantage lies in the quantity, not quality, of its arms. “Military clashes between South and North Korea over the past years in West Sea have proven that the conventional weapons equipment performance of NK is inferior to that of ROK,” Dr. Sungpyo Hong of Ajou University reports. “Altogether, the ROK is superior to the North in conventional weapons and equipment in general.”

The DPRK has roughly twice the number of men under arms, nearly 50 percent more main battle tanks, and twice as many artillery pieces. Rolling that mass southward would do damage but would not conquer the South. Pyongyang’s greatest advantage is defensive. In any war the DPRK could wreck Seoul—which lies some 25 miles from the border—with artillery and SCUD missiles, a very high price for the ROK to pay even for victory.

But here is where the conundrum comes in: why does the ROK continue to lag behind the North in any measure of military power? The disparity in numbers is not due to circumstances beyond Seoul’s control. There is no special geographical feature that ensures, say, that there will always be fewer men under arms in the southern half of the peninsula. Rather, the South Korean government doesn’t want to spend more money to defend itself.

Over the last decade, according to Dr. Ho, the “ROK military has decreased its troops from 690,000 to 650,000” even though the North had more than a million men in uniform. Seoul cannot complain about the resulting numerical disparity.

Bruce Klingner of the Heritage Foundation points to Seoul’s Defense Reform Plan 2020, adopted in 2005, which planned to cut total military manpower from 681,000 to 500,000. Nothing has changed in the years since, even after repeated North Korean provocations, including the sinking of an ROK warship and bombardment of an ROK island in 2010.

Apparently South Koreans aren’t worried about their defense. Or they assume they can rely on Americans to protect them with whatever force is necessary.

Yet America’s foreign-policy community seems oblivious to the perverse incentives of military welfare. It is widely accepted that generous social welfare in the U.S. long discouraged work, marriage, and education: this realization drove the 1996 federal welfare reform legislation.

Washington’s military welfare for foreign nations has a similarly debilitating impact. Even while relying on America for defense from North Korea, the ROK began fashioning a blue-water navy capable of conducting more distant missions. And Seoul spent a decade actually subsidizing the DPRK, as part of the so-called “Sunshine Policy.”

Colonial Americans secured their homeland before embarking on foreign adventures. They certainly didn’t expect Great Britain, France, Germany, or some other nation to protect them for decades so they could, in the words of Klingner, “assume a greater role on the world stage that is commensurate with” their growing capabilities. Washington should welcome South Korea’s emergence as a genuine global power. But that should not mean subsidizing South Koreans’ pursuit of foreign aggrandizement.

The emergence of a prosperous and democratic South Korea has benefited the U.S. and the rest of the world—it’s one of the great post-World War II success stories. Americans have special reason to be satisfied, since Washington’s defense shield enabled the ROK to develop despite North Korea’s threats.

But the South no longer needs U.S. support, which by now is only a source of military unpreparedness, the root of the Korean conundrum. Peoples of the two nations should remain friends—cultural, family, and economic ties do not depend on military deployments. And the two governments should cooperate in areas of shared political and military interest. But it is high time for Seoul to shift from security dependent to security adult and solve its strategic conundrum once and for all.

Doug Bandow is a senior fellow at the Cato Institute.