Article 51 of the United Nations Charter has acknowledged the right of self-defence as an inherent right of a state if an armed attack occurs against that state. However, a right of self-defence is not clear in the scope of its application.
Some scholars argue that the relevant article of the United Nations Charter limits the scope of the right of the self-defence recognized under the customary international law, but it is difficult to agree with the view in light of the judgment delivered by the International Court of Justice in the 1986 Nicaragua case. Today, even under the UN Charter system, the right of anticipatory self-defense should be admitted at least in case of a nuclear attack with the strict conditions. Otherwise, the attacked state which lose defence capabilities due to such an unexpected nuclear attack could not make counter-strike against the attacking state.
However, it is difficult to accept the legal justification of a military response of a state to terrorist attacks as self-defence based on “a pattern of attacks” argument. Meanwhile, the act of sinking Cheonanham (Cheonan vessel) by the North Korean torpedo attack could constitute an armed attack provided for Article 51 of the Charter. Notwithstanding the fact, it's inappropriate for South Korea to exercise a right of self-defence in the present case. That's why the sinking is a by-gone blow.
The use of force of a punitive nature ex post facto should be regarded as armed reprisal or military retorsion rather than a legitimate exercise of self-defence. Such an armed response is not allowed according to the principles of peaceful settlement of international disputes under Article 2 Paragraph 3 of the United Nations Charter. Of course, it's exceptional in cases where UN Security Council authorizes the use of force expressly or implicitly. But it may be hard for us to expect the authorization of the UN Security Counci in the Cheonanham case. Therefore, it is reasonable for South Korea to submit the case to the UN Security Council in close cooperation with the countries concerned and seek a peaceful solution by taking advantage of the United Nations' mechanism.