The law of neutrality is not obsolete. Its validity subsists despite the prohibition of the use of force,the advent of collective security and the violations of the rights of neutrals during the two world wars. The sources of the law are customary laws and treaties. Customary law of neutrality is based on State practice mainly in the form of national military manuals. It is applicable in the event of an international armed conflict irrespective of the existence of a formal state of war and irrespective of the scale and extent of th conflict. Neutrality has withstood the challenge of collective security and the prohibition of the use of force both in the interwar years and after World War II. The gross violations of the rights of neutrals during World Wars I and II have been justified as reprisals or countermeasures; this is a further confirmation that beligerent practice at the time constituted a deviation from the law rather than a change or demise of the law of neutrality.