The practice of declaring war has a long history. The ancient Sumerian Epic of Gilgamesh gives an account of it,[7] as does the Old Testament.[8][9]
However, the practice of declaring war was not always strictly followed. In his study Hostilities without Declaration of War (1883), the British scholar John Frederick Maurice showed that between 1700 and 1870 war was declared in only 10 cases, while in another 107 cases war was waged without such declaration (these figures include only wars waged in Europe and between European states and the United States, not including colonial wars in Africa and Asia).
In modern public international law, a declaration of war entails the recognition between countries of a state of hostilities between these countries, and such declaration has acted to regulate the conduct between the military engagements between the forces of the respective countries. The primary multilateral treaties governing such declarations are the Hague Conventions.
The League of Nations, formed in 1919 in the wake of the First World War, and the General Treaty for the Renunciation of War of 1928 signed in Paris, France, demonstrated that world powers were seriously seeking a means to prevent the carnage of another world war. Nevertheless, these powers were unable to stop the outbreak of the Second World War, so the United Nations (UN) was established following that war in a renewed attempt to prevent international aggression through declarations of war.
http://en.wikipedia.org/wiki/Declaration_of_war在英国学者John Frederick Maurice 的《不宣而战》（1883年出版）这本书里讨论了1700-1870年间欧洲和美洲国家的117次战争，其中只有10次正式宣战，剩余的107次没有正式宣战。
这段里还提到对于宣战作为双方正式进入交战状态的条件，是在海牙会议（1899和1907年）和1928年巴黎条约里规定的。
