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Nearly a month has passed since the Navy corvette Cheonan sank, but regrettably, we have produced controversies and speculation about the cause of the tragedy, rather than proof. But what has come to light makes it increasingly obvious that the sinking was caused not by an internal malfunction but by an external attack. If that is indeed the case, what should the government have done, and what actions should it take from now on?

In international law, the United Nations Charter’s Article 51 provides that a state has the “inherent right of individual or collective self-defense if an armed attack occurs.” The right of self-defense was first exercised by the United States in 1837, when the British attacked the American steamer Caroline, which...