North Korea's announcement that it had successfully tested a nuclear device has renewed concerns that other countries in the region will follow suit. These are not idle concerns. Given Japan's existing nuclear power program, and its advanced technical and industrial base, it could likely develop nuclear weapons in a matter of months. But it is far from certain that Japan will go that route, and Prime Minister Shinzo Abe was quick to dampen speculation about an ensuing arms race. "Possession of nuclear arms is not an option at all for our country," Mr. Abe said after the North Korean test....