Is International Law Effective PART 2: When can states use force?

By Aoibhín Spriggs - Law Student @ St Catharine’s College, Cambridge


The prohibition on the use of force is a cornerstone of international law, codified in Article 2(4) of the United Nations Charter:

“The Organization and its Members, in pursuit of the Purposes stated in Article 1, shall act in accordance with the following Principles:

[...] 4. All Members shall refrain in their international relations from the threat or use of force against territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.”

However, there are exceptions to this prohibition for United Nations Security Council authorisations, invitation, and the right to self-defence.

United Nations Security Council Authorisation

Article 1(1) of the UN Charter sets out the purpose of the UN Charter to maintain international peace and security and do so by taking effective collective measures. The states agree to give the power to make decisions on the use of force to the Security Council. Coercive measures must be taken under chapter 7. First, the UNSC determines if there exists any threat to the peace, breach of peace, or act of aggression and makes a recommendation on this basis. It then can make a non-forcible measure or a forcible measure. Non-forcible measures include sanctions, such as the economic sanctions in place against Russia. If these are judged to be inadequate, the UNSC may recommend forcible measures, including operations by air, sea, or land forces of members of the UN.

Intervention by invitation

Intervention at the request of the government does not breach Article 2(4) as it is not ‘against the territorial integrity or political independence' of the state, for example, if President Zelensky asked states to intervene to help with the Ukrainian efforts. However, the group that is invited to another state must be legitimate - so a government installed by a foreign power, would not be able to invite. Given that this would only be used when there are great difficulties in the country and the situation is tumultuous, the application in practice may be deceptively difficult.

Right to Self-Defence

The right to self-defence is found in Article 51 of the UN Charter. For the right to self-defence to be triggered, there must be not just the use of force, but an armed attack against a state.

This is supplemented by customary international law, which requires two conditions to act in self-defence. First, the action taken must be necessary- this means that there is no alternative, that the measures of law enforcement would not be sufficient. Second, the force must be proportionate to the attack, and with the purpose of self-defence in repelling this attack.

A state may also commit an indirect armed attack by sending armed bands to the territory of another state, or by a substantial involvement. However, a mere supply of funds, although an act of intervention, does not in itself amount to a use of force. For example, many Western states have sent tanks, military supplies, and funds to Ukraine in its fight against Russia - although this is an act of intervention, it does not violate Article 2(4). If, however, these states chose to send armies, arm the Ukrainian army or give them logistical support this would constitute an armed attack.


United Nations (UN): an intergovernmental organisation, set up after WW2 with the aim of preventing future wars. It currently has 193 member states

United Nations Charter: the foundational treaty of the UN, establishing the purpose, governing structure and overall framework of the UN System.

United Nations Security Council (UNSC): one of the principal organs of the UN. It has fifteen members, five of which are permanent (China, France, Russia, UK, USA) and rotating ten members serving a term of two years.

Customary International Law: A source of international law, which is a general practice accepted as law by most states and is not necessarily written or codified. States must do something or refrain from doing something, on the basis that they feel they are legal required to act this way.

International Court of Justice (ICJ): the principal judicial organ of the UN, composed of 15 judges who are elected for terms of nine years. The court’s role is to settle legal disputes submitted to it by states.

Further reading:

On the Nicaragua v USA case:

For a good summary of the legal issues:

or the press summary on the ICJ website:

On Russia’s use of force in Ukraine: