The law of neutrality was mainly developed during the 19th century. The most important factors in its development were the two Armed Neutralities of 1780 and 1800, the attitude of the United States towards neutrality, the permanent neutrality of Switzerland and Belgium and the Declaration of Paris of 1856. At the Second Hague Peace Conference the subject seemed to be ready for codification. In spite of being to a large extent declaratory of existing international law, the two Conventions on neutrality concluded at The Hague were not ratified by Great Britain, Italy and some other States.
Apart from these two conventions, other Conventions concluded in 1907 are also important for the law of neutrality, particularly the Convention (VII) relating to the Conversion of Merchant Ships into War-Ships, the Convention (XI) relative to Certain Restrictions with regard to the Exercise of the Right of Capture in Naval War and the Convention (XII) relative to the Creation of an International Prize Court. Furthermore, the unratified Declaration of London of 1909 contained the most important rules concerning neutrality in naval war.
As a consequence of the two World Wars, the prohibition of the resort to war and the systems of collective security established by the League of Nations and the United Nations, the traditional law of neutrality has lost much of its former importance.