International law permits one state to use force against another state only in certain narrowly defined situations. A state may use force in selfdefense in the event of an "armed attack" by another state. Additionally, treaties may give a state the right to use force in designated situations. Apart from these exceptions, however, the prohibition against the use of force is quite strict.
The Legality of the United States Invasion of Panama,
Yale J. Int'l L.
Available at: http://digitalcommons.law.yale.edu/yjil/vol15/iss2/3