North Korea is threatening to fight back.
Although Seoul says it has no intention of stopping and boarding a North Korean vessel, Pyongyang responded in harsh tones Wednesday to South Korea's joining the Proliferation Security Initiative as a full-fledged member.
A North Korean military spokesman declared that "search and seizure" of a North Korean vessel would be "an unpardonable infringement on our sovereignty" to which "we will immediately respond with a powerful military strike."
North Korea tested a nuclear weapon Monday and has test-fired half a dozen missiles this week. These actions, coinciding with South Korean reports that North Korea has restarted its main nuclear facility, prompted South Korea to join the global counter-proliferation program, known as the Proliferation Security Initiative (PSI).
The global program was initiated during the presidency of George W. Bush in 2003 to draw nations together to halt the flow of nuclear materiel and of missiles for firing warheads to distant targets. South Korea's decision makes it one of 15 core members of PSI. Previously it had been an observer among scores of nations participating on an occasional basis.
South Korea's foreign minister, Yu Hwang Hwan, calls the decision to join the PSI "a natural obligation for a mature country" to "help control North Korea's development of dangerous materiel."
Walking away from reconciliation
South Korea's move may not have an immediate impact but counts as a milestone in a rising confrontation. Its membership in PSI is seen by some analysts as an acknowledgement of the end of efforts at reconciliation with the North after the failure of six-party talks to get North Korea to abandon its nuclear weapons program.
For now, South Korea's membership only carries symbolic significance – unless or until the United States specifically asks South Korea to assist in blockading ships suspected of carrying illicit cargo.
In practical terms, the PSI has been effective mainly as a vehicle for exchanging information and staging exercises.
US offers warm welcome into PSI
The speed and enthusiasm with which the Obama administration greeted South Korea's decision to join PSI suggests that the US may want to use PSI as more than a vehicle for training exercises and exchange of information on the movement of suspicious vessels and cargo.
President Obama personally thanked South Korean President Lee Myung Bak for joining PSI in a telephone conversation. White House spokesman Robert Gibbs said the US looks forward to "working with the South Korean Government to stop the proliferation of WMD-related materials worldwide and to strengthening the Initiative for the future."
The next important date, after joining PSI, will be the first use of PSI as the basis for actually stopping a North Korean ship.
South Korea may have had the opportunity last month when a North Korean freighter asked for assistance against marauding pirates off the Somali coast. A South Korean destroyer in the area sent a Lynx helicopter to scare off the pirates. Next time, say analysts, the South Koreans may want to board the North Korean vessel and see what it's carrying.
PSI cooperation may boost alliances
It's closer to home, though, that PSI may have its biggest impact.
South Korea's membership "plugs an obvious gap," says Bruce Klinger, senior research fellow on Northeast Asia at the conservative Heritage Foundation here. "It's really an enforcement mechanism. If North Korea doesn't try to proliferate, you don't need it."
South Korean membership in PSI may, however, elevate confrontation not just with North Korea but with China, on which North Korea depends for fuel, food, and fertilizer. "China is not participating," Mr. Klinger points out, while "Japan would be a partner."
"It means heightened cooperation among the US, Japan, and South Korea," he says. "It puts the US, Japan, and South Korea in a much stronger position vis-à-vis China."
While South Korea earlier appeared "a little iffy" about PSI, says Mr. Flake, "North Korea's nuclear test "provided a gift-wrapped opportunity." The PSI "ultimately will be more important than anything else we can do to change North Korea's purpose" in becoming a nuclear power, he continues.
South Korea, reluctant to anger North Korea, participated only as an observer in PSI exercises while Roh Moo Hyun, the South's president from 2003 to 2008, pursued reconciliation with the North.
Mr. Roh's conservative successor, President Lee, declared the South's "firm and clear" intention of joining PSI so after the North test-fired a long-range missile on April 5 but held back on a final decision.