In the event of airport closures, the best option for Japanese citizens to return home would be by sea from the southeastern port city of Busan. The Japanese government is working to obtain cooperation from U.S. forces stationed in South Korea to transport evacuees across the country from Seoul to Busan.
The Japanese Self-Defense Forces would need permission from South Korea's government to operate inside the country. Approval has not been forthcoming and could provoke a backlash from a South Korean public harboring historical grievances at the former colonial power. But SDF vessels could help in ferrying Japanese citizens home from Busan.
Such a crises could make it easier for terrorists and other dangerous individuals to enter Japan disguised as returning citizens. The Japanese government aims to work with the U.S. to prevent such unlawful entry. One proposal would create a temporary holding area for returnees in Busan or Japan.
"We are looking at a range of responses" to a crisis on the Korean Peninsula, from securing evacuees and processing their entry to creating and operating holding facilities, as well as determining whether Japan is responsible for their protection, Abe had said at a parliamentary session in April.
According to Nikkei, there are currently about 38,000 long-term Japanese residents in South Korea, as well as another 19,000 or so tourists and other short-term travelers. "If the U.S. decided on a military strike against the North, the Japanese government would start moving toward an evacuation on its own accord regardless of whether the American plans are public," a Japanese government source said.
If I read between the lines of this article, I see where the Japanese and possibly S. Korean governments think the U.S. will tell them ahead of time that a strike on the North is forthcoming. LOLOLOL!!!
Since there won't be much left of either country after the dust settles, I don't think the U.S. really cares, do you?