The United States adopted the so-called 'Patriot Act', an anti-terrorism law aimed at making the investigations in cases of terrorism easier. The United Kingdom adopted an anti-terror bill with the same purpose. The United Nations Security Council has adopted several resolutions on the matter and American states signed the Inter-American Convention against terrorism on 3 June 2002. This chapter analyses the use of force by states against terrorist groups operating from other state's territory and its effect on the concept of state sovereignty. Keywords: international law; state sovereignty; terrorism