text: The only requirement is that some sort of ultimatum be issued prior to the declaration of war.\n\nOpening of Hostilities (Hague III); October 18, 1907\nCONVENTION RELATIVE TO THE OPENING OF HOSTILITIES \nArticle 1\nThe Contracting Powers recognize that hostilities between themselves must not commence without previous and explicit warning, in the form either of a reasoned declaration of war or of an ultimatum with conditional declaration of war.