A declaration of war is a formal act by which one state announces existing or impending war activity against another. The declaration is a performative speech act by an authorized party of a national government, in order to create a state of war between two or more states.

The Hague Conventions of 1899 and 1907 are a series of international treaties and declarations negotiated at two international peace conferences at The Hague in the Netherlands. Along with the Geneva Conventions, the Hague Conventions were among the first formal statements of the laws of war and war crimes in the body of secular international law. A third conference was planned for 1914 and later rescheduled for 1915, but it did not take place because of the start of World War I.