A brief description of the use of force in self-defence

Before considering whether the right to exercise self-defence justifies any measures taken by a State, let's take at look at the general rule of international law regarding the use of force. The general prohibition of the use of force is provided by Article 2 para 4 of the UN Charter. Two exceptions to this prohibition are (1) authorization by the UN Security Council (Article 42 UN Charter) and (2) self-defence (Article 51 of the UN Charter).
         As can be learned from the Advisory Opinion on the Threat and Use of Nuclear Weapons from 8 July 1996, paragraph 41, the submission of exercise of the right of self-defence to conditions of necessity and proportionality, is a rule of customary international law. Referring to the Nicaragua case, the ICJ states that "there is a specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it, a rule well established in customary international law", see also paragraph 51 and paras 73-77 of the Oil Platforms case. The Oil Platforms case also makes clear that the principle of necessity is a strict and objective condition that leaves States no discretion.
      
Thus, the three requirements to invoke self-defence, (known as the "Caroline test") as can be derived from the 1842 letter from Webster to Ashburton, are: the other state must have performed an armed attack (1), self-defence was necessary to respond to it (2) and the self-defence was proportionate (3) (see also, paras 229-237 Nicaragua). Moreover, Art. 51 of the UN Charter requires that measures taken shall be reported to the Security Council immediately.

Different regimes for the UN Charter and customary international law (Caroline test)?
While measures taken in either reactive or anticipatory self-defence would have to comply with the requirements of necessity and proportionality, Art. 51 of the UN Charter stipulates that self-defence can only be invoked after the occurrence of an armed attack.
        The Caroline test, formulating the requirements of customary international law, leaves the possibility that one invokes a state of necessity if an imminent threat has been established, the necessity of the self-defence is instant and a state has no other means left to it than a use of force. Both the necessity and proportionality principle imply that states have to confine themselves to repelling the attack(er)- self-defence cannot be invoked in order to pursue or impose punishment on the attacker. 

Note that time is another condition of importance, as in the Nicaragua case, the measures taken several months after the major offense, could not be said to "correspond to a necessity justifying the action" (para 237).
      Asssuming that any measures taken have not yet failed the necessity test, the next question is, if the proportionality principle has been met. The Court states in the Oil Platforms case that "the scale of the whole operation has to be judged in order to assess, whether the response ["self-defence"] was proportionate" (para 77).
      A problem arises when an attack was performed by a non-state actor [rebel groups, for instance]. Is their conduct attributable to a state? A strict, traditionalist approach was applied in Nicaragua (demanding "effective control", paras 115-116), subsequently in Armed Activities on the Territory of The Congo (paras 146, 147) and the Genocide case of 2007 (Bosnia and Herzegovina vs. Serbia and Montenegro, para 406).
     Provided that a state has delivered arms to a rebel group (Nicaragua) and although providing training and military support to a rebel group might violate customary international law (Armed Activities Congo/ Uganda), the conclusion that follows from these cases is that State could not be held responsible and that the self-defence plea had to be rejected for these reasons.
     Not everything has been said yet. A lot of controversy still surrounds the question, whether the right to self-defence could be invoked against non-state actors, assuming that their conduct cannot be attributable to a state. It is rather unsure whether or not the traditionalist approach still applies. Hence, in practice, the right to invoke self-defence against non-state actors is no longer categorically rejected.