Terrorism in all its aspects is a criminal act under International Law.
Art.51 says that "Nothing in the present charter shall impair the inherent right of individual or collective self defense if an armed attack occurs against a Member of United Nations, until the Security Council has taken measures necessary to maintain international peace and security." Let us examine some cases which define the international Law standard for whether a particular use of force is self defense.
American ship, The Caroline in a US port; because the Caroline had been used in American raids into Canadian territory. The British claimed the attack was a self defense. But the dispute was resolved in favor of Americans.
This particular case had given a clear meaning to the term self defense under International Law as "there must be a necessity of self defense- instant, overwhelming, leaving no choice of means and no choice of means and no moment for deliberation." The means of self defense must involve nothing unreasonable or excessive: Since the act, justified by the necessity of self defense, must be limited by that necessity, and kept clearly within it. This has been accepted rule in International Law. This is some what called as "anticipatory Self defense".
The representive of Nicaragua Vs The United states of America