4. Second exception to the prohibition on the use of force: Right of self defense

Note: This blog post should not be considered a comprehensive or an updated analysis on the subject. This was used as teaching material to supplement classes conducted in 2010.

International law recognizes an individual right of self defence (“victim” State against the “aggressor” State) and collective right of self defense (“victim” State + friendly State/s against the “aggressor” State). The UN Charter and customary international law acknowledge the right to use force in self defense.

For example, Article 51 recognizes the inherent right of a member State to use force in self defense when an armed attack occurs against that State.

“Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council…”


What is an armed attack and who can carry out an armed attack?

In Nicaragua case, ICJ says an armed attack is: (1) action by regular State armed forces across an international border; (2) armed groups, irregular forces and mercenaries when (a) they are “sent by or on behalf of a State” to carry out an armed attack against another State and (b) the attack is of such gravity  so that it amounts to an armed attack if it was conducted by regular armed forces of a State.

In the DRC vs Uganda case the ICJ held that Uganda had not proved that it was subjected to an armed attack by the DRC.

The traditional view is that State “B” does not have a right of self defense against State “A”: if armed groups within State “A” carried out an armed attack against State “B”; unless, these rebels was sent by or on behalf of State “A”.

The ICJ in the DRC vs Uganda case declined to comment on whether and under what conditions contemporary international law provides for a right of self-defense against large-scale attacks by irregular forces. The ICJ, nevertheless, observed that “…the taking of airports and towns many hundreds of kilometers from Uganda’s border would not seem proportionate to the series of trans-border attacks it claimed had given rise to the right of self-defense, nor to be necessary to that end.”

Read paras 193-5 of the Nicaragua Case; paras 131-135 and 146 and 147 of the DRC Case; para 139 of the Palestinian Wall Case Advisory Opinion.


What is not an armed attack according to the ICJ in the Nicaragua case?

If State “A” supplies of weapons and logistical to a rebel group, which the rebel groups use to attack State “B” – can the supply of weapons and logistical support be considered as an armed attack by State “A” against the State “B”?

In Nicaragua Case the ICJ said no. It held that this may amount to a threat or use of force or illegal intervention in the affairs of another State but it was not an armed attack. This means that State “B” does not have the right of self defense against State “A” under Article 51 of the Charter because an armed attack has not occurred. Do you agree? Read Judge Jenning’s dissenting opinion in Nicaragua Case. Does this reading into an armed attack mean that if State “A” illegally uses force against State “B” (remember that use of force is both direct and indirect and can include the supply of weapons to rebels) and as a result State “A” violates Article 2(4); Article 51 does not give State “B” a right to use force in self defense and that State “A” can violate Article 2(4) with impunity? The Court said that instead of relying on self defense State B can take “proportional countermeasures” against State A in such a situation. ✐ What proportional countermeasures can State B take? Do you think this is enough?

Mere frontier incidents are not “armed attacks” if the necessary “scale and effects” are not there.

When can a State use force in self defense according to Article 51?

An armed attack has to have occurred against a member State (read the section before on armed attack and the situation in which a State can use self defense). (See the section below on the possibility to use self defense even if an armed attack has not occurred or where it has occurred against a national of a State. )

self defense is only available against the aggressor State (the one who carried out or on whose behalf an armed attack was carried out) by the victim State (subject of the attack). Read para 139 of the Palestinian Wall Case Advisory Opinion. Do you agree?

The only way a third State will have a right of self defense against the aggressor State is if the victim State asks for the help of the third State (we call this collective self defense). Read the facts of the Nicaragua case.

Any use of force in self defense must be necessary and proportionate to the armed attack. Read para.176 of the Nicaragua Case and paras 41-48 of the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion of the ICJ (1996).

As we discussed a State that uses force in self defense must immediately inform the SC and this State can use force only until the SC steps in. See both DRC vs Uganda and Nicaragua vs USA.

What we discussed so far is the treaty or UN Charter right to self defense. In addition to the treaty right, some argue that there is also a customary international law right to self defense. They argue that the Charter never intended to restrict the customary international law right of self defense (which is more wider than the right under A. 51) and that the reference to the “inherent right” of self defense in Article 51 brings in the customary international law right of self defense into Article 51. Read the other arguments put forward by Dixon, p. 297 (5th Ed.).


© Ruwanthika Gunaratne and Public International Law at https://ruwanthikagunaratne.wordpress.com, 2008 – 2018. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Ruwanthika Gunaratne and Public International Law with appropriate and specific direction to the original content.


  1. I believe anticipatory self defense should be incorporated in article 51 and thus self defense should not only be defensive but first strike as well so long as it satisfies ingredients of self defense

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