There exist today a number of conventions aimed at combating terrorism. These treat acts of terrorism as criminal acts. However, the events of 11 September 2001 introduced a new dimension into the debate on the use of force in addressing the problem of terrorism. This article discusses whether the UN Security Council has given its approval for the USA’s use of force in the wake of 11 September 2001 and whether this use of force can be justified under the right of self-defence. The article’s conclusion is that the Security Council has not given its approval. Nevertheless, the USA may invoke the right of self-defence on the basis of the Taliban’s support for the terrorists. However, it is important to remember that acts of terrorism ought as far as possible to be addressed through criminal prosecution. Furthermore, any use of force ought to take place under the control of the UN. And we must be on our guard against any erosion of the prohibition against the use of force in international law.