While the prohibition on the use of force between states has been recognized a jus cogens, there is still a legal framework for the use of force in the international law. However, the legality of the use of force has been challenged given the recent use of force by states in response to attacks which were not carried out by other states. Among other issues, the legality of the use of force against terrorists who are non state actors has been much discussed. Given the rapid outbreak of terrorism over the world, the international community has witneesed a development on international use of force against terrorists both in terms of international law and states practice. Yet there are ongoing debates as whether states can use force against terrorists operated in another state, where the terrorist acts are not attributable to states.
To address the above matter, the present thesis focuses on the legality of the use of force against terrorists whose conducts are not attributable to states. This thesis shows that there is a complexity in international law which challenges the use of force against non-state actors which are terrorists in particular and where the terrorist acts are not attributable to states. As far as the international law is concerned, it is no doubt that the settlement of such complexities has to be complied with the international law. However, by and large, the states practice during the past decades appears to gradually recognize the right of states to unilaterally use force against terrorists. In doing so, to certain extent, the conditions for the acceptable forcible responses to terrorism are likely to be approached by a more lenient way, notwithstanding that it shall be compatible with the United Nations Charter regime.
On the basic of the legal rules governing the use of force, and in order to render a visual assessment on the use of force by a state against terrorists operated in other states, an analysis on the United Kingdom of Great Britain and Northern Ireland's use of force against the Islamic State in Syria starting in December 2015 is taken. This is so to determine whether the airstrikes of the United Kingdom were consistent with the international law on the use of force.
This thesis is expected to contribute an overview on recent development of the international use of force against terrorists where their conducts are not attributable to states, and to partly show a sound and clearer substantive ground on the use of force against terrorists in international relation.