North Korea's fateful decision to test a nuclear bomb makes it the ninth and least welcome of the nations believed to possess such weapons. If verified, the test announced in triumph by Pyongyang presents a grave threat to global security that should not - and will not - go unanswered.

The risks are clear. Japan, nervous about North Korean missiles capable of hitting Japanese soil, will be tempted to develop its own nuclear arsenal, a move that would abruptly end two generations of Japanese pacifism and stoke an east Asian arms race. Aspiring nuclear powers, including Iran, will be emboldened by the defiant stance of Kim Jong-il, the North Korean leader.