Time and time again, the principle of self-defense in international law has come under serious threats and tests.
Recently, Gaza had fired missiles into Isreali territory. Isreal reacts with attacks on Gaza, saying it attacked Gaza in self-defense as it possesses that right under International law. Again, did the massive U.S. attack on Afghanistan in response to 9/11 go too far in the context of self-defense? Or more recently in Libya, were there circumstances in which Libya might have invoked its right to self-defense to resist military action taken against the state pursuant to a Security Council authorisation?
Historically, the right of states to take up arms to defend themselves from external force is well established as a rule of customary International law. Article 51 of the United Nations Charter 1945 however often to the inherent right of collective self-defense and the question therefore arises as to how far one state may resort to force in the defense of another?
The idea of collective self-defense however is rather ambiguous. It may be regarded merely as a pulling of a number of individual right of self-defense within the framework of a particular treaty or in situation; or it may form the basis of a comprehensive regional security system if the former were the case.
From a practical point of view, it might lead to legal difficulties if for instance, Iceland should resort to force in defense of Nigerian interest since action against Nigeria would in no way justify an armed reaction by Iceland pursuant to its individual right of self-defense.
In fact, state practice has adopted the second approach. Organisations such as NATOand the WARSAR Pactwere established after the Second World War, specifically based on the right of collective self-defense under Article 51. By such agreements, an attack upon one party is an attack upon all.
The International Court of Justice (ICJ) in the celebrated Nicaragua casehas however clearly established that the right of self-defense exists as an inherent right under customary International law as well as the under the UN Charter. For its judgment based on the circum stances of the particular case, in connection with the military and para-military activities against Nicaragua by the U.S., the international court by twelve votes to three rejected the justification of collective self-defense maintained by the United States of America in that case.
In conclusion, the provisions governing the resort to force in internationally do not affect the right of a state to take measures to maintain order within its jurisdiction. Article 51 of the UN Charter does not limit the circumstances in which self-defense may be exercised.
 An alliance of countries from North America and Europe committed to fulfilling the goals of the North Atlantic Treaty signed on 4 April 1949.
 The Warsaw Pact was the Soviet reply to NATO. The Warsaw Pact was domintaed by the Soviet Union. It is commonly given to the treaty between Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, Romania, and the Soviet Union, which was signed in Poland in 1955.
 The Republic of Nicaragua v. The United States of America (1984) ICJ REP. 392