I would assume that the South Korean government was expecting this kind of criticism from their political opposition and had a quick response available. There is just no way that the US military would bring Japanese troops on to the peninsula without the direct approval of the ROK government:
South Korea’s Chief Defense Minister Han Min-koo (Yonhap)
South Korea is capable of turning down any U.S. request for the entry of Japanese armed forces onto the Korean Peninsula, Defense Minister Han Min-koo said Monday in his latest efforts to defuse concerns over Japan’s rearmament moves.
Japan passed a package of security bills into law in the wee hours of Saturday, unshackling its previously self defense-only armed forces from limitations imposed after World War II and empowering them to fight in overseas battles.
The rearmament step has sparked concerns here that Japanese forces could join U.S. troops stationed in South Korea unilaterally in the event of a war with North Korea.
Rep. Lee Choon-suk of the main opposition New Politics Alliance for Democracy raised the issue of unwanted Japanese forces during a parliamentary audit, asking Han if the country could turn down the U.S. military’s request for the entry of Japanese forces in the event of a North Korea-waged war. [Korea Times]
