The prohibition of violence in Article 2 (4) of the UN Charter is today considered a cornerstone of international law. The prohibition is universal and has only two exceptions: the right of states to self-defence against armed attack by another state (Article 51) and with the authorization of the Security Council (Article 42).

A novel situation is when a non-state actor (e.g. a terrorist organization), operating in the territory of one country, directs armed attacks from that country against another country. When the 'host' country is unwilling or unable to prevent the attacks, it is not uncommon for the attacked state to invoke the 'unwillingness or inability' argument as a legal basis for using force in self-defense. Russia's use of force in Georgia in 2002, Uganda in Congo in 2003, Turkey in Iraq and Syria in 2015-2018 are some examples. The use of force is directed against a non-state actor, but leads to the violation of the territorial integrity of the 'host' country.

In two high-profile cases in 2004 and 2005, the International Court of Justice ruled that the prohibition of violence in Article 2(4) remains in force and that the fight against international terrorism has not changed the legal situation. State practice shows the opposite. International law doctrine is divided on the issue. The purpose of this project is to examine the background and development of the concept of "unwilling or unable states" and its status in contemporary international law. The ultimate aim of the study is to examine whether a new legal basis for the use of force in inter-state relations may be emerging.