South Korean nuclear weapons would be a step further, though they are far from outside the realm of possibility. South Korea has a nuclear power program and could probably pull together the expertise to begin a nuclear weapons program in a relatively short period.
The idea has significant support within South Korea. Won Yoo-chul, a senior figure in the then-ruling conservative party, said last year that his country should develop “peaceful” nuclear weapons to counteract North Korea's “fearful and self-destructive” ones. A few months later, a poll found that 60 percent of the nation supported such a move — a figure in keeping with most polls conducted in South Korea since 2006.
There may even be considerable international support for such a move: Before he became president, Donald Trump raised the possibility of nations like South Korea and Japan having their own nuclear weapons. “We have a nuclear world now,” Trump said in March 2016.
But many analysts are deeply concerned by the prospect of a South Korean nuclear program — and worry that it may add to a problem it claims to resolve.
The history of South Korea's nuclear ambitions
It's not quite a nuclear world yet, no matter what the U.S. president says. It's thought that nine countries have successfully tested a hydrogen bomb — and North Korea is the only country to have done so in the past two decades.
South Korea did once hope to be a member of this club. In the early 1970s, as its nuclear power program grew, President Park Chung-hee began to investigate possibilities of nuclear weapons. Declassified documents show that the U.S. government was deeply concerned about South Korea, then a military dictatorship, having access to nuclear weapons and pressured it to abandon those ambitions.
South Korea eventually ratified the nuclear Non-Proliferation Treaty (NPT) in April 1975, in large part because of pressure from the United States and Canada — a move that, in theory, forced it to drop its program.
However, it is widely believed that Seoul pursued nuclear weapons covertly in later years. In 2004, the country acknowledged secret nuclear experiments to the International Atomic Energy Agency (IAEA). Partly because of such experiments, some military experts suggest that South Korea might be able to develop a nuclear weapon in as short a time as 18 months, the conservative newspaper Chosun Ilbo reported last year.
Even without nuclear weapons, South Korea technically has a nuclear deterrence.
Since the Korean War, the country has been under the protection of the U.S. nuclear umbrella — an assurance that it would be protected by U.S. nuclear weapons if needed. That safeguard remains even though the United States moved its nuclear weapons out of South Korea in 1991 as part of a bid to persuade North Korea to allow the IAEA to inspect that country's nuclear sites. At the time, Pyongyang and Seoul also jointly committed to making the peninsula free of nuclear weapons.
Some recent reports have also suggested that the Trump administration had considered reinstalling U.S. nuclear weapons in South Korea.
Talk of a restart
Talk of a South Korean nuclear weapons program is nothing new. Analysts have often warned against it for several reasons.
Perhaps most immediately, if South Korea were to restart its nuclear weapons program, it would mean withdrawing from the NPT. The list of countries that have done this is short and, as Troy Stangarone of the Korea Economic Institute of America put it in the National Interest last year, it's “an ignominious club”: North Korea is the only other nation to have done so.
Such a move could tarnish South Korea's standing on the world stage and perhaps lead to sanctions. It could also cause problems for its domestic nuclear power industry, which is estimated to account for 30 percent of the country's total consumption.
Worse still, some fear that a South Korean nuclear program could spur a broader regional arms race, with Japan or Taiwan perhaps developing nuclear weapons or China expanding its program. This would add an unpredictability to the region and probably undermine stability.
Even on the Korean Peninsula, it's easy to see the drawbacks. Faced with the threat of South Korea developing nuclear weapons, the North may be incentivized to launch a preemptive strike. The risk of miscalculation by either side would grow dangerously.
It's probably because of these arguments that previous pushes for South Korean nuclear weapons have fallen flat. When the issue flared up again last year, then-President Park Geun-hye dismissed the idea. Park's successor, Moon Jae-in, has pushed for peace talks with the North as opposed to military solutions to the conflict.
Even the high poll numbers in support of nuclear weapons are disputed. Robert Einhorn and Duyeon Kim wrote in the Bulletin of the Atomic Scientists last year that many experts discount them as South Koreans are often surveyed immediately after a North Korean test, a time when tensions remain high.
Instead, many say that talk of South Korean weapons is simply designed to pressure other nations — most obviously, the United States and China. “I don’t think that South Korea actually wants nuclear weapons,” Park Syung-je, chairman of the Asia Strategy Institute in Seoul, recently told The Washington Post. “It’s a way of saying to the Chinese that ‘if you don’t cooperate on North Korea, then we’re going to get nuclear weapons of our own.’”
But the frame of the debate over North Korea's weapons program has evolved dramatically in the recent months. Attempts to force China to pressure its belligerent neighbor have produced, at best, mixed results. And with the increasing likelihood that North Korea — theoretically — may soon be able to hit a major U.S. city with a nuclear weapon, it's no surprise that South Koreans wonder how protected they really are.