text: what criteria should be satisfied before one country declares war on another?   the only requirement is that some sort of ultimatum be issued prior to the declaration of war.  opening of hostilities (hague iii); october 18, 1907 convention relative to the opening of hostilities  article 1 the 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.