As Edward Snowden knows better than anyone, intelligence agencies are usually tasked with keeping, rather than divulging, secrets. But the National Intelligence Service, South Korea's spy agency, has triggered an unholy political row after disclosing a classified transcript of the 2007 inter-Korean summit.

For months, ruling conservative New Frontier Party lawmakers have been on the warpath over allegations that former South Korean President Roh Moo-hyun agreed with then North Korean leader Kim Jong Il at the meeting to revise...